diff --git a/integration-tests/plugins/__tests__/regions/admin/regions.spec.ts b/integration-tests/plugins/__tests__/regions/admin/regions.spec.ts
index f55d6f17ec..33862a1a22 100644
--- a/integration-tests/plugins/__tests__/regions/admin/regions.spec.ts
+++ b/integration-tests/plugins/__tests__/regions/admin/regions.spec.ts
@@ -52,6 +52,8 @@ describe("Regions - Admin", () => {
       {
         name: "Test Region",
         currency_code: "usd",
+        countries: ["us", "ca"],
+        metadata: { foo: "bar" },
       },
       adminHeaders
     )
@@ -62,14 +64,21 @@ describe("Regions - Admin", () => {
         id: created.data.region.id,
         name: "Test Region",
         currency_code: "usd",
+        metadata: { foo: "bar" },
       })
     )
+    expect(created.data.region.countries.map((c) => c.iso_2)).toEqual([
+      "us",
+      "ca",
+    ])
 
     const updated = await api.post(
-      `/admin/regions`,
+      `/admin/regions/${created.data.region.id}`,
       {
         name: "United States",
         currency_code: "usd",
+        countries: ["us"],
+        metadata: { foo: "baz" },
       },
       adminHeaders
     )
@@ -78,9 +87,12 @@ describe("Regions - Admin", () => {
     expect(updated.data.region).toEqual(
       expect.objectContaining({
         id: updated.data.region.id,
+        name: "United States",
         currency_code: "usd",
+        metadata: { foo: "baz" },
       })
     )
+    expect(updated.data.region.countries.map((c) => c.iso_2)).toEqual(["us"])
 
     const deleted = await api.delete(
       `/admin/regions/${updated.data.region.id}`,
@@ -129,7 +141,7 @@ describe("Regions - Admin", () => {
 
     expect(error.response.status).toEqual(400)
     expect(error.response.data.message).toEqual(
-      "Currency with code: foo was not found"
+      'Currencies with codes: "foo" were not found'
     )
   })
 
@@ -180,6 +192,8 @@ describe("Regions - Admin", () => {
       {
         name: "Test",
         currency_code: "usd",
+        countries: ["jp"],
+        metadata: { foo: "bar" },
       },
     ])
 
@@ -192,8 +206,12 @@ describe("Regions - Admin", () => {
         id: expect.any(String),
         name: "Test",
         currency_code: "usd",
+        metadata: { foo: "bar" },
       }),
     ])
+    expect(response.data.regions[0].countries.map((c) => c.iso_2)).toEqual([
+      "jp",
+    ])
   })
 
   it("should get a region", async () => {
@@ -201,6 +219,8 @@ describe("Regions - Admin", () => {
       {
         name: "Test",
         currency_code: "usd",
+        countries: ["jp"],
+        metadata: { foo: "bar" },
       },
     ])
 
@@ -213,7 +233,9 @@ describe("Regions - Admin", () => {
         id: region.id,
         name: "Test",
         currency_code: "usd",
+        metadata: { foo: "bar" },
       })
     )
+    expect(response.data.region.countries.map((c) => c.iso_2)).toEqual(["jp"])
   })
 })
diff --git a/packages/core-flows/src/region/steps/update-regions.ts b/packages/core-flows/src/region/steps/update-regions.ts
index 1e30f1344c..daec4ad790 100644
--- a/packages/core-flows/src/region/steps/update-regions.ts
+++ b/packages/core-flows/src/region/steps/update-regions.ts
@@ -48,6 +48,7 @@ export const updateRegionsStep = createStep(
         name: r.name,
         currency_code: r.currency_code,
         metadata: r.metadata,
+        countries: r.countries.map((c) => c.iso_2),
       }))
     )
   }
diff --git a/packages/medusa/src/api-v2/admin/regions/validators.ts b/packages/medusa/src/api-v2/admin/regions/validators.ts
index 96ef914928..8fd977f5d9 100644
--- a/packages/medusa/src/api-v2/admin/regions/validators.ts
+++ b/packages/medusa/src/api-v2/admin/regions/validators.ts
@@ -1,6 +1,12 @@
 import { OperatorMap } from "@medusajs/types"
 import { Type } from "class-transformer"
-import { IsOptional, IsString, ValidateNested } from "class-validator"
+import {
+  IsArray,
+  IsObject,
+  IsOptional,
+  IsString,
+  ValidateNested,
+} from "class-validator"
 import { FindParams, extendedFindParamsMixin } from "../../../types/common"
 import { OperatorMapValidator } from "../../../types/validators/operator-map"
 
@@ -76,6 +82,14 @@ export class AdminPostRegionsReq {
 
   @IsString()
   currency_code: string
+
+  @IsArray()
+  @IsOptional()
+  countries?: string[]
+
+  @IsObject()
+  @IsOptional()
+  metadata?: Record<string, unknown>
 }
 
 export class AdminPostRegionsRegionReq {
@@ -86,4 +100,12 @@ export class AdminPostRegionsRegionReq {
   @IsString()
   @IsOptional()
   currency_code?: string
+
+  @IsArray()
+  @IsOptional()
+  countries?: string[]
+
+  @IsObject()
+  @IsOptional()
+  metadata?: Record<string, unknown>
 }
diff --git a/packages/region/integration-tests/__tests__/region-module.spec.ts b/packages/region/integration-tests/__tests__/region-module.spec.ts
index cf43c02bee..42c8a365cd 100644
--- a/packages/region/integration-tests/__tests__/region-module.spec.ts
+++ b/packages/region/integration-tests/__tests__/region-module.spec.ts
@@ -127,7 +127,7 @@ describe("Region Module Service", () => {
         currency_code: "USD",
         countries: ["neverland"],
       })
-    ).rejects.toThrowError("Countries with codes neverland does not exist")
+    ).rejects.toThrowError('Countries with codes: "neverland" do not exist')
   })
 
   it("should throw when country is already assigned to a region", async () => {
@@ -144,7 +144,7 @@ describe("Region Module Service", () => {
         countries: ["us"],
       })
     ).rejects.toThrowError(
-      "Country with code us is already assigned to a region"
+      'Countries with codes: "us" are already assigned to a region'
     )
   })
 
@@ -163,7 +163,163 @@ describe("Region Module Service", () => {
         },
       ])
     ).rejects.toThrowError(
-      "Country with code us is already assigned to a region"
+      'Countries with codes: "us" are already assigned to a region'
+    )
+  })
+
+  it("should update the region successfully", async () => {
+    const createdRegion = await service.create({
+      name: "North America",
+      currency_code: "USD",
+      countries: ["us", "ca"],
+    })
+
+    await service.update(createdRegion.id, {
+      name: "Americas",
+      currency_code: "MXN",
+      countries: ["us", "mx"],
+    })
+
+    const latestRegion = await service.retrieve(createdRegion.id, {
+      relations: ["currency", "countries"],
+    })
+
+    expect(latestRegion).toMatchObject({
+      id: createdRegion.id,
+      name: "Americas",
+      currency_code: "mxn",
+    })
+
+    expect(latestRegion.countries.map((c) => c.iso_2)).toEqual(["mx", "us"])
+  })
+
+  it("should update the region without affecting countries if countries are undefined", async () => {
+    const createdRegion = await service.create({
+      name: "North America",
+      currency_code: "USD",
+      countries: ["us", "ca"],
+    })
+
+    await service.update(createdRegion.id, {
+      name: "Americas",
+      currency_code: "MXN",
+    })
+
+    const updatedRegion = await service.retrieve(createdRegion.id, {
+      relations: ["currency", "countries"],
+    })
+
+    expect(updatedRegion).toMatchObject({
+      id: createdRegion.id,
+      name: "Americas",
+      currency_code: "mxn",
+    })
+
+    expect(updatedRegion.countries.map((c) => c.iso_2)).toEqual(["ca", "us"])
+  })
+
+  it("should remove the countries in a region successfully", async () => {
+    const createdRegion = await service.create({
+      name: "North America",
+      currency_code: "USD",
+      countries: ["us", "ca"],
+    })
+
+    await service.update(createdRegion.id, {
+      name: "Americas",
+      currency_code: "MXN",
+      countries: [],
+    })
+
+    const updatedRegion = await service.retrieve(createdRegion.id, {
+      relations: ["currency", "countries"],
+    })
+
+    expect(updatedRegion).toMatchObject({
+      id: createdRegion.id,
+      name: "Americas",
+      currency_code: "mxn",
+    })
+
+    expect(updatedRegion.countries).toHaveLength(0)
+  })
+
+  it("should fail updating the region currency to a non-existent one", async () => {
+    const createdRegion = await service.create({
+      name: "North America",
+      currency_code: "USD",
+      countries: ["us", "ca"],
+    })
+
+    await expect(
+      service.update(
+        { id: createdRegion.id },
+        {
+          currency_code: "DOGECOIN",
+        }
+      )
+    ).rejects.toThrowError('Currencies with codes: "dogecoin" were not found')
+  })
+
+  it("should fail updating the region countries to non-existent ones", async () => {
+    const createdRegion = await service.create({
+      name: "North America",
+      currency_code: "USD",
+      countries: ["us", "ca"],
+    })
+
+    await expect(
+      service.update(
+        { id: createdRegion.id },
+        {
+          countries: ["us", "neverland"],
+        }
+      )
+    ).rejects.toThrowError('Countries with codes: "neverland" do not exist')
+  })
+
+  it("should fail updating the region if there are duplicate countries", async () => {
+    const createdRegion = await service.create({
+      name: "North America",
+      currency_code: "USD",
+      countries: ["us", "ca"],
+    })
+
+    await expect(
+      service.update(
+        { id: createdRegion.id },
+        {
+          countries: ["us", "us"],
+        }
+      )
+    ).rejects.toThrowError(
+      'Countries with codes: "us" are already assigned to a region'
+    )
+  })
+
+  it("should fail updating the region if country is already used", async () => {
+    const [createdRegion] = await service.create([
+      {
+        name: "North America",
+        currency_code: "USD",
+        countries: ["us", "ca"],
+      },
+      {
+        name: "Americas",
+        currency_code: "USD",
+        countries: ["mx"],
+      },
+    ])
+
+    await expect(
+      service.update(
+        { id: createdRegion.id },
+        {
+          countries: ["us", "mx"],
+        }
+      )
+    ).rejects.toThrowError(
+      'Countries with codes: "mx" are already assigned to a region'
     )
   })
 
@@ -173,6 +329,6 @@ describe("Region Module Service", () => {
         name: "Europe",
         currency_code: "DOGECOIN",
       })
-    ).rejects.toThrowError("Currency with code: DOGECOIN was not found")
+    ).rejects.toThrowError('Currencies with codes: "dogecoin" were not found')
   })
 })
diff --git a/packages/region/src/migrations/.snapshot-medusa-region.json b/packages/region/src/migrations/.snapshot-medusa-region.json
index 83539e71fc..4aeb715cba 100644
--- a/packages/region/src/migrations/.snapshot-medusa-region.json
+++ b/packages/region/src/migrations/.snapshot-medusa-region.json
@@ -135,7 +135,7 @@
       "indexes": [
         {
           "columnNames": [
-            "currency_id"
+            "currency_code"
           ],
           "composite": false,
           "keyName": "IDX_region_currency_code",
@@ -163,7 +163,7 @@
       ],
       "checks": [],
       "foreignKeys": {
-        "region_currency_id_foreign": {
+        "region_currency_code_foreign": {
           "constraintName": "region_currency_code_foreign",
           "columnNames": [
             "currency_code"
@@ -173,7 +173,7 @@
             "code"
           ],
           "referencedTableName": "public.region_currency",
-          "deleteRule": "cascade",
+          "deleteRule": "set null",
           "updateRule": "cascade"
         }
       }
@@ -247,15 +247,6 @@
       "name": "region_country",
       "schema": "public",
       "indexes": [
-        {
-          "columnNames": [
-            "region_id"
-          ],
-          "composite": false,
-          "keyName": "IDX_country_region_id",
-          "primary": false,
-          "unique": false
-        },
         {
           "keyName": "region_country_pkey",
           "columnNames": [
@@ -278,10 +269,10 @@
             "id"
           ],
           "referencedTableName": "public.region",
-          "deleteRule": "cascade",
+          "deleteRule": "set null",
           "updateRule": "cascade"
         }
       }
     }
   ]
-}
\ No newline at end of file
+}
diff --git a/packages/region/src/migrations/RegionModuleSetup20240205173216.ts b/packages/region/src/migrations/RegionModuleSetup20240205173216.ts
index c60010a279..71546c8106 100644
--- a/packages/region/src/migrations/RegionModuleSetup20240205173216.ts
+++ b/packages/region/src/migrations/RegionModuleSetup20240205173216.ts
@@ -51,7 +51,7 @@ CREATE TABLE IF NOT EXISTS "region_country" (
   "region_id" text NULL,
   CONSTRAINT "region_country_pkey" PRIMARY KEY ("id")
 );
-CREATE INDEX IF NOT EXISTS "IDX_country_region_id" ON "region_country" ("region_id");
+CREATE UNIQUE INDEX IF NOT EXISTS "IDX_region_country_region_id_iso_2_unique" ON "region_country" (region_id, iso_2);
 
 -- Adjust foreign keys for "region_country"
 ALTER TABLE "region_country" DROP CONSTRAINT IF EXISTS "FK_91f88052197680f9790272aaf5b";
diff --git a/packages/region/src/models/country.ts b/packages/region/src/models/country.ts
index 9df8d781c6..65a565149f 100644
--- a/packages/region/src/models/country.ts
+++ b/packages/region/src/models/country.ts
@@ -1,15 +1,30 @@
 import {
   BeforeCreate,
   Entity,
+  Unique,
   ManyToOne,
   OnInit,
   PrimaryKey,
   Property,
+  Index,
 } from "@mikro-orm/core"
 
-import { generateEntityId } from "@medusajs/utils"
+import {
+  createPsqlIndexStatementHelper,
+  generateEntityId,
+} from "@medusajs/utils"
 import Region from "./region"
 
+// We don't need a partial index on deleted_at here since we don't support soft deletes on countries
+const regionIdIsoIndexName = "IDX_region_country_region_id_iso_2_unique"
+const RegionIdIsoIndexStatement = createPsqlIndexStatementHelper({
+  name: regionIdIsoIndexName,
+  tableName: "region_country",
+  columns: ["region_id", "iso_2"],
+  unique: true,
+})
+
+RegionIdIsoIndexStatement.MikroORMIndex()
 @Entity({ tableName: "region_country" })
 export default class Country {
   @PrimaryKey({ columnType: "text" })
@@ -35,7 +50,6 @@ export default class Country {
 
   @ManyToOne({
     entity: () => Region,
-    index: "IDX_country_region_id",
     nullable: true,
   })
   region?: Region | null
diff --git a/packages/region/src/services/region-module.ts b/packages/region/src/services/region-module.ts
index 485565ddb9..ad779ee581 100644
--- a/packages/region/src/services/region-module.ts
+++ b/packages/region/src/services/region-module.ts
@@ -14,6 +14,7 @@ import {
   UpdateRegionDTO,
 } from "@medusajs/types"
 import {
+  arrayDifference,
   InjectManager,
   InjectTransactionManager,
   isObject,
@@ -27,11 +28,7 @@ import {
 import { Country, Currency, Region } from "@models"
 
 import { DefaultsUtils } from "@medusajs/utils"
-import {
-  CreateCountryDTO,
-  CreateCurrencyDTO,
-  CreateRegionDTO as InternalCreateRegionDTO,
-} from "@types"
+import { CreateCountryDTO, CreateCurrencyDTO } from "@types"
 import { entityNameToLinkableKeysMap, joinerConfig } from "../joiner-config"
 
 const COUNTRIES_LIMIT = 1000
@@ -117,78 +114,31 @@ export default class RegionModuleService<
     data: CreateRegionDTO[],
     @MedusaContext() sharedContext: Context = {}
   ): Promise<Region[]> {
-    let currencies = await this.currencyService_.list(
-      { code: data.map((d) => d.currency_code.toLowerCase()) },
-      {},
-      sharedContext
+    let normalizedInput = RegionModuleService.normalizeInput(data)
+
+    const validations = [
+      this.validateCurrencies(
+        normalizedInput.map((r) => r.currency_code),
+        sharedContext
+      ),
+      this.validateCountries(
+        normalizedInput.map((r) => r.countries ?? []).flat(),
+        sharedContext
+      ),
+    ] as const
+
+    // Assign the full country object so the ORM updates the relationship
+    const [, dbCountries] = await promiseAll(validations)
+    const dbCountriesMap = new Map(dbCountries.map((d) => [d.iso_2, d]))
+    let normalizedDbRegions = normalizedInput.map((region) =>
+      removeUndefined({
+        ...region,
+        countries: region.countries?.map((c) => dbCountriesMap.get(c)),
+      })
     )
 
-    const countriesInDb = await this.countryService_.list(
-      { iso_2: data.map((d) => d.countries).flat() },
-      { select: ["iso_2", "region_id"] },
-      sharedContext
-    )
-
-    const countriesInDbMap = new Map<string, Country>(
-      countriesInDb.map((c) => [c.iso_2, c])
-    )
-
-    const currencyMap = new Map(
-      currencies.map((c) => [c.code.toLowerCase(), c])
-    )
-
-    const toCreate: InternalCreateRegionDTO[] = []
-    const seenCountries = new Set<string>()
-    for (const region of data) {
-      const reg = { ...region } as InternalCreateRegionDTO
-      const countriesToAdd = region.countries || []
-
-      if (countriesToAdd) {
-        const notInDb = countriesToAdd.filter(
-          (c) => !countriesInDbMap.has(c.toLowerCase())
-        )
-
-        if (notInDb.length) {
-          throw new MedusaError(
-            MedusaError.Types.INVALID_DATA,
-            `Countries with codes ${countriesToAdd.join(", ")} does not exist`
-          )
-        }
-
-        const regionCountries = countriesToAdd.map((code) => {
-          const country = countriesInDbMap.get(code.toLowerCase())
-
-          if (country?.region_id || seenCountries.has(code.toLowerCase())) {
-            throw new MedusaError(
-              MedusaError.Types.INVALID_DATA,
-              `Country with code ${code} is already assigned to a region`
-            )
-          }
-
-          seenCountries.add(code.toLowerCase())
-
-          return country
-        }) as unknown as TCountry[]
-
-        reg.countries = regionCountries
-      }
-
-      const lowerCasedCurrency = region.currency_code.toLowerCase()
-      if (!currencyMap.has(lowerCasedCurrency)) {
-        throw new MedusaError(
-          MedusaError.Types.INVALID_DATA,
-          `Currency with code: ${region.currency_code} was not found`
-        )
-      }
-
-      reg.currency_code = lowerCasedCurrency
-
-      toCreate.push(reg as InternalCreateRegionDTO)
-    }
-
-    const result = await this.regionService_.create(toCreate, sharedContext)
-
-    return result
+    // Create the regions and update the country region_id
+    return await this.regionService_.create(normalizedDbRegions, sharedContext)
   }
 
   async update(
@@ -208,11 +158,15 @@ export default class RegionModuleService<
     data?: UpdatableRegionFields,
     @MedusaContext() sharedContext: Context = {}
   ): Promise<RegionDTO | RegionDTO[]> {
-    const result = await this.update_(idOrSelectorOrData, data, sharedContext)
+    const updateResult = await this.update_(
+      idOrSelectorOrData,
+      data,
+      sharedContext
+    )
 
     const regions = await this.baseRepository_.serialize<
       RegionDTO[] | RegionDTO
-    >(result)
+    >(updateResult)
 
     return isString(idOrSelectorOrData) ? regions[0] : regions
   }
@@ -223,37 +177,171 @@ export default class RegionModuleService<
     data?: UpdatableRegionFields,
     @MedusaContext() sharedContext: Context = {}
   ): Promise<Region[]> {
-    let result: Region[] = []
+    let normalizedInput: UpdateRegionDTO[] = []
     if (isString(idOrSelectorOrData)) {
-      result = await this.regionService_.update(
-        [{ id: idOrSelectorOrData, ...data }],
-        sharedContext
-      )
+      normalizedInput = [{ id: idOrSelectorOrData, ...data }]
     }
 
     if (Array.isArray(idOrSelectorOrData)) {
-      result = await this.regionService_.update(
-        idOrSelectorOrData,
-        sharedContext
-      )
+      normalizedInput = idOrSelectorOrData
     }
 
     if (isObject(idOrSelectorOrData)) {
-      let toUpdate: Partial<UpdateRegionDTO>[] = []
       const regions = await this.regionService_.list(
-        { ...idOrSelectorOrData },
+        idOrSelectorOrData,
         {},
         sharedContext
       )
 
-      regions.forEach((region) => {
-        toUpdate.push({ id: region.id, ...data })
-      })
+      normalizedInput = regions.map((region) => ({
+        id: region.id,
+        ...data,
+      }))
+    }
+    normalizedInput = RegionModuleService.normalizeInput(normalizedInput)
 
-      result = await this.regionService_.update(toUpdate, sharedContext)
+    // If countries are being updated for a region, first make previously set countries' region to null to get to a clean slate.
+    // Somewhat less efficient, but region operations will be very rare, so it is better to go with a simple solution
+    const regionsWithCountryUpdate = normalizedInput
+      .filter((region) => !!region.countries)
+      .map((region) => region.id)
+      .flat()
+
+    if (regionsWithCountryUpdate.length) {
+      await this.countryService_.update(
+        {
+          selector: {
+            region_id: regionsWithCountryUpdate,
+          },
+          data: { region_id: null },
+        },
+        sharedContext
+      )
     }
 
-    return result
+    const validations = [
+      this.validateCurrencies(
+        normalizedInput.map((d) => d.currency_code),
+        sharedContext
+      ),
+      this.validateCountries(
+        normalizedInput.map((d) => d.countries ?? []).flat(),
+        sharedContext
+      ),
+    ] as const
+
+    // Assign the full country object so the ORM updates the relationship
+    const [, dbCountries] = await promiseAll(validations)
+    const dbCountriesMap = new Map(dbCountries.map((d) => [d.iso_2, d]))
+    let normalizedDbRegions = normalizedInput.map((region) =>
+      removeUndefined({
+        ...region,
+        countries: region.countries?.map((c) => dbCountriesMap.get(c)),
+      })
+    )
+
+    return await this.regionService_.update(normalizedDbRegions, sharedContext)
+  }
+
+  private static normalizeInput<T extends UpdatableRegionFields>(
+    regions: T[]
+  ): T[] {
+    return regions.map((region) =>
+      removeUndefined({
+        ...region,
+        currency_code: region.currency_code?.toLowerCase(),
+        name: region.name?.trim(),
+        countries: region.countries?.map((country) => country.toLowerCase()),
+      })
+    )
+  }
+
+  private async validateCurrencies(
+    currencyCodes: (string | undefined)[] | undefined,
+    sharedContext: Context
+  ): Promise<void> {
+    const normalizedCurrencyCodes = currencyCodes
+      ?.filter((c) => c !== undefined)
+      .map((c) => c!.toLowerCase())
+
+    if (!normalizedCurrencyCodes?.length) {
+      return
+    }
+
+    const uniqueCurrencyCodes = Array.from(new Set(normalizedCurrencyCodes))
+    const dbCurrencies = await this.currencyService_.list(
+      { code: uniqueCurrencyCodes },
+      {},
+      sharedContext
+    )
+    const dbCurrencyCodes = dbCurrencies.map((c) => c.code.toLowerCase())
+
+    if (uniqueCurrencyCodes.length !== dbCurrencyCodes.length) {
+      const missingCurrencies = arrayDifference(
+        uniqueCurrencyCodes,
+        dbCurrencyCodes
+      )
+
+      throw new MedusaError(
+        MedusaError.Types.INVALID_DATA,
+        `Currencies with codes: "${missingCurrencies.join(
+          ", "
+        )}" were not found`
+      )
+    }
+  }
+
+  private async validateCountries(
+    countries: string[] | undefined,
+    sharedContext: Context
+  ): Promise<TCountry[]> {
+    if (!countries?.length) {
+      return []
+    }
+
+    // The new regions being created have a country conflict
+    const uniqueCountries = Array.from(new Set(countries))
+    if (uniqueCountries.length !== countries.length) {
+      throw new MedusaError(
+        MedusaError.Types.INVALID_DATA,
+        `Countries with codes: "${getDuplicateEntries(countries).join(
+          ", "
+        )}" are already assigned to a region`
+      )
+    }
+
+    const countriesInDb = await this.countryService_.list(
+      { iso_2: uniqueCountries },
+      { select: ["iso_2", "region_id"] },
+      sharedContext
+    )
+    const countryCodesInDb = countriesInDb.map((c) => c.iso_2.toLowerCase())
+
+    // Countries missing in the database
+    if (countriesInDb.length !== uniqueCountries.length) {
+      const missingCountries = arrayDifference(
+        uniqueCountries,
+        countryCodesInDb
+      )
+
+      throw new MedusaError(
+        MedusaError.Types.INVALID_DATA,
+        `Countries with codes: "${missingCountries.join(", ")}" do not exist`
+      )
+    }
+
+    // Countries that already have a region already assigned to them
+    const countriesWithRegion = countriesInDb.filter((c) => !!c.region_id)
+    if (countriesWithRegion.length) {
+      throw new MedusaError(
+        MedusaError.Types.INVALID_DATA,
+        `Countries with codes: "${countriesWithRegion
+          .map((c) => c.iso_2)
+          .join(", ")}" are already assigned to a region`
+      )
+    }
+
+    return countriesInDb
   }
 
   @InjectManager("baseRepository_")
@@ -325,3 +413,25 @@ export default class RegionModuleService<
     }
   }
 }
+
+// microORM complains if undefined fields are present in the passed data object
+const removeUndefined = <T extends Record<string, any>>(obj: T): T => {
+  return Object.fromEntries(
+    Object.entries(obj).filter(([_, v]) => v !== undefined)
+  ) as T
+}
+
+const getDuplicateEntries = (collection: string[]): string[] => {
+  const uniqueElements = new Set()
+  const duplicates: string[] = []
+
+  collection.forEach((item) => {
+    if (uniqueElements.has(item)) {
+      duplicates.push(item)
+    } else {
+      uniqueElements.add(item)
+    }
+  })
+
+  return duplicates
+}
diff --git a/packages/region/src/types/index.ts b/packages/region/src/types/index.ts
index c1841fd471..e4f28fb1c6 100644
--- a/packages/region/src/types/index.ts
+++ b/packages/region/src/types/index.ts
@@ -24,9 +24,3 @@ export type CreateCountryDTO = {
   name: string
   display_name: string
 }
-
-export type CreateRegionDTO = {
-  name: string
-  currency_code: string
-  countries?: Country[]
-}
\ No newline at end of file
diff --git a/packages/types/src/region/mutations.ts b/packages/types/src/region/mutations.ts
index 88ad80be69..1a2ff1c55e 100644
--- a/packages/types/src/region/mutations.ts
+++ b/packages/types/src/region/mutations.ts
@@ -7,21 +7,15 @@ export interface CreateRegionDTO {
 
 export interface UpdateRegionDTO {
   id: string
+  name?: string
   currency_code?: string
   countries?: string[]
-  name?: string
   metadata?: Record<string, unknown>
 }
 
 export interface UpdatableRegionFields {
-  currency_code?: string
   name?: string
+  currency_code?: string
+  countries?: string[]
   metadata?: Record<string, unknown>
 }
-
-export interface AddCountryToRegionDTO {
-  region_id: string
-  country_id: string
-}
-
-export interface RemoveCountryFromRegionDTO extends AddCountryToRegionDTO {}