diff --git a/packages/next/src/build/index.ts b/packages/next/src/build/index.ts
index 01be33f2b85b7..37b9f15438a17 100644
--- a/packages/next/src/build/index.ts
+++ b/packages/next/src/build/index.ts
@@ -377,13 +377,13 @@ export type RoutesManifest = {
   dynamicRoutes: Array<ManifestRoute>
   dataRoutes: Array<ManifestDataRoute>
   i18n?: {
-    domains?: Array<{
+    domains?: ReadonlyArray<{
       http?: true
       domain: string
-      locales?: string[]
+      locales?: readonly string[]
       defaultLocale: string
     }>
-    locales: string[]
+    locales: readonly string[]
     defaultLocale: string
     localeDetection?: false
   }
@@ -473,7 +473,11 @@ async function writeClientSsgManifest(
     buildId,
     distDir,
     locales,
-  }: { buildId: string; distDir: string; locales: string[] }
+  }: {
+    buildId: string
+    distDir: string
+    locales: readonly string[] | undefined
+  }
 ) {
   const ssgPages = new Set<string>(
     [
@@ -3622,7 +3626,7 @@ export default async function build(
         await writeClientSsgManifest(prerenderManifest, {
           distDir,
           buildId,
-          locales: config.i18n?.locales || [],
+          locales: config.i18n?.locales,
         })
       } else {
         await writePrerenderManifest(distDir, {
diff --git a/packages/next/src/build/static-paths/pages.ts b/packages/next/src/build/static-paths/pages.ts
index 3a5ad406cbc79..b684006fc95b3 100644
--- a/packages/next/src/build/static-paths/pages.ts
+++ b/packages/next/src/build/static-paths/pages.ts
@@ -19,7 +19,7 @@ export async function buildPagesStaticPaths({
   page: string
   getStaticPaths: GetStaticPaths
   configFileName: string
-  locales?: string[]
+  locales?: readonly string[]
   defaultLocale?: string
 }): Promise<StaticPathsResult> {
   const prerenderedRoutes: PrerenderedRoute[] = []
@@ -28,7 +28,13 @@ export async function buildPagesStaticPaths({
 
   // Get the default list of allowed params.
   const routeParameterKeys = Object.keys(_routeMatcher(page))
-  const staticPathsResult = await getStaticPaths({ locales, defaultLocale })
+  const staticPathsResult = await getStaticPaths({
+    // We create a copy here to avoid having the types of `getStaticPaths`
+    // change. This ensures that users can't mutate this array and have it
+    // poison the reference.
+    locales: [...(locales ?? [])],
+    defaultLocale,
+  })
 
   const expectedReturnVal =
     `Expected: { paths: [], fallback: boolean }\n` +
diff --git a/packages/next/src/build/utils.ts b/packages/next/src/build/utils.ts
index f9b78d79ea77a..ad6c70ce3d297 100644
--- a/packages/next/src/build/utils.ts
+++ b/packages/next/src/build/utils.ts
@@ -981,7 +981,7 @@ export async function isPageStatic({
   configFileName: string
   runtimeEnvConfig: any
   httpAgentOptions: NextConfigComplete['httpAgentOptions']
-  locales?: string[]
+  locales?: readonly string[]
   defaultLocale?: string
   parentId?: any
   edgeInfo?: any
diff --git a/packages/next/src/client/get-domain-locale.ts b/packages/next/src/client/get-domain-locale.ts
index 0924cb34b7779..aebe4ccec95fe 100644
--- a/packages/next/src/client/get-domain-locale.ts
+++ b/packages/next/src/client/get-domain-locale.ts
@@ -8,8 +8,8 @@ const basePath = (process.env.__NEXT_ROUTER_BASEPATH as string) || ''
 export function getDomainLocale(
   path: string,
   locale?: string | false,
-  locales?: string[],
-  domainLocales?: DomainLocale[]
+  locales?: readonly string[],
+  domainLocales?: readonly DomainLocale[]
 ) {
   if (process.env.__NEXT_I18N_SUPPORT) {
     const normalizeLocalePath: typeof NormalizeFn =
diff --git a/packages/next/src/server/accept-header.ts b/packages/next/src/server/accept-header.ts
index 071c48737ae1c..38284657914df 100644
--- a/packages/next/src/server/accept-header.ts
+++ b/packages/next/src/server/accept-header.ts
@@ -12,7 +12,7 @@ interface Options {
 
 function parse(
   raw: string,
-  preferences: string[] | undefined,
+  preferences: readonly string[] | undefined,
   options: Options
 ) {
   const lowers = new Map<string, { orig: string; pos: number }>()
@@ -127,7 +127,7 @@ function parse(
   return preferred
 }
 
-export function acceptLanguage(header = '', preferences?: string[]) {
+export function acceptLanguage(header = '', preferences?: readonly string[]) {
   return (
     parse(header, preferences, {
       type: 'accept-language',
diff --git a/packages/next/src/server/base-server.ts b/packages/next/src/server/base-server.ts
index 3c23f8af11522..564ffc68f4aec 100644
--- a/packages/next/src/server/base-server.ts
+++ b/packages/next/src/server/base-server.ts
@@ -1437,7 +1437,7 @@ export default abstract class Server<
         }
         const normalizeResult = normalizeLocalePath(
           removePathPrefix(parsedUrl.pathname, this.nextConfig.basePath || ''),
-          this.nextConfig.i18n?.locales || []
+          this.nextConfig.i18n?.locales
         )
 
         if (normalizeResult.detectedLocale) {
diff --git a/packages/next/src/server/config-shared.ts b/packages/next/src/server/config-shared.ts
index 026f9c93fee64..629fb1e740c82 100644
--- a/packages/next/src/server/config-shared.ts
+++ b/packages/next/src/server/config-shared.ts
@@ -23,20 +23,20 @@ export type NextConfigComplete = Required<NextConfig> & {
   configFileName: string
 }
 
-export type I18NDomains = DomainLocale[]
+export type I18NDomains = readonly DomainLocale[]
 
 export interface I18NConfig {
   defaultLocale: string
   domains?: I18NDomains
   localeDetection?: false
-  locales: string[]
+  locales: readonly string[]
 }
 
 export interface DomainLocale {
   defaultLocale: string
   domain: string
   http?: true
-  locales?: string[]
+  locales?: readonly string[]
 }
 
 export interface ESLintConfig {
diff --git a/packages/next/src/server/dev/static-paths-worker.ts b/packages/next/src/server/dev/static-paths-worker.ts
index 17b1330aebf53..c1978b515f6fc 100644
--- a/packages/next/src/server/dev/static-paths-worker.ts
+++ b/packages/next/src/server/dev/static-paths-worker.ts
@@ -55,7 +55,7 @@ export async function loadStaticPaths({
   pathname: string
   config: RuntimeConfig
   httpAgentOptions: NextConfigComplete['httpAgentOptions']
-  locales?: string[]
+  locales?: readonly string[]
   defaultLocale?: string
   isAppPath: boolean
   page: string
diff --git a/packages/next/src/server/render.tsx b/packages/next/src/server/render.tsx
index b2022d743f53d..ed33dde5a8c3a 100644
--- a/packages/next/src/server/render.tsx
+++ b/packages/next/src/server/render.tsx
@@ -142,9 +142,9 @@ class ServerRouter implements NextRouter {
   isFallback: boolean
   locale?: string
   isReady: boolean
-  locales?: string[]
+  locales?: readonly string[]
   defaultLocale?: string
-  domainLocales?: DomainLocale[]
+  domainLocales?: readonly DomainLocale[]
   isPreview: boolean
   isLocaleDomain: boolean
 
@@ -156,9 +156,9 @@ class ServerRouter implements NextRouter {
     isReady: boolean,
     basePath: string,
     locale?: string,
-    locales?: string[],
+    locales?: readonly string[],
     defaultLocale?: string,
-    domainLocales?: DomainLocale[],
+    domainLocales?: readonly DomainLocale[],
     isPreview?: boolean,
     isLocaleDomain?: boolean
   ) {
@@ -261,9 +261,9 @@ export type RenderOptsPartial = {
   nextFontManifest?: DeepReadonly<NextFontManifest>
   distDir?: string
   locale?: string
-  locales?: string[]
+  locales?: readonly string[]
   defaultLocale?: string
-  domainLocales?: DomainLocale[]
+  domainLocales?: readonly DomainLocale[]
   disableOptimizedLoading?: boolean
   supportsDynamicResponse: boolean
   isBot?: boolean
@@ -848,7 +848,7 @@ export async function renderToHTMLImpl(
             ...(isPreview
               ? { draftMode: true, preview: true, previewData: previewData }
               : undefined),
-            locales: renderOpts.locales,
+            locales: [...(renderOpts.locales ?? [])],
             locale: renderOpts.locale,
             defaultLocale: renderOpts.defaultLocale,
             revalidateReason: renderOpts.isOnDemandRevalidate
@@ -1071,7 +1071,10 @@ export async function renderToHTMLImpl(
             ...(previewData !== false
               ? { draftMode: true, preview: true, previewData: previewData }
               : undefined),
-            locales: renderOpts.locales,
+            // We create a copy here to avoid having the types of
+            // `getServerSideProps` change. This ensures that users can't
+            // mutate this array and have it poison the reference.
+            locales: [...(renderOpts.locales ?? [])],
             locale: renderOpts.locale,
             defaultLocale: renderOpts.defaultLocale,
           })
diff --git a/packages/next/src/server/require.ts b/packages/next/src/server/require.ts
index fda15e7c5e855..95e4e22a599e7 100644
--- a/packages/next/src/server/require.ts
+++ b/packages/next/src/server/require.ts
@@ -19,7 +19,7 @@ const pagePathCache = !isDev ? new LRUCache<string | null>(1000) : null
 export function getMaybePagePath(
   page: string,
   distDir: string,
-  locales: string[] | undefined,
+  locales: readonly string[] | undefined,
   isAppPath: boolean
 ): string | null {
   const cacheKey = `${page}:${distDir}:${locales}:${isAppPath}`
diff --git a/packages/next/src/shared/lib/i18n/detect-domain-locale.ts b/packages/next/src/shared/lib/i18n/detect-domain-locale.ts
index a7ce3e07a0cd5..df9d9f35a5e88 100644
--- a/packages/next/src/shared/lib/i18n/detect-domain-locale.ts
+++ b/packages/next/src/shared/lib/i18n/detect-domain-locale.ts
@@ -1,7 +1,7 @@
 import type { DomainLocale } from '../../../server/config-shared'
 
 export function detectDomainLocale(
-  domainItems?: DomainLocale[],
+  domainItems?: readonly DomainLocale[],
   hostname?: string,
   detectedLocale?: string
 ) {
diff --git a/packages/next/src/shared/lib/i18n/normalize-locale-path.ts b/packages/next/src/shared/lib/i18n/normalize-locale-path.ts
index d687605e14a28..146b854b35652 100644
--- a/packages/next/src/shared/lib/i18n/normalize-locale-path.ts
+++ b/packages/next/src/shared/lib/i18n/normalize-locale-path.ts
@@ -14,7 +14,7 @@ export interface PathLocale {
  */
 export function normalizeLocalePath(
   pathname: string,
-  locales?: string[]
+  locales?: readonly string[]
 ): PathLocale {
   let detectedLocale: string | undefined
   // first item will be empty string from splitting at first char
diff --git a/packages/next/src/shared/lib/router/router.ts b/packages/next/src/shared/lib/router/router.ts
index e74ae88268b16..5d0fa2b7eed4f 100644
--- a/packages/next/src/shared/lib/router/router.ts
+++ b/packages/next/src/shared/lib/router/router.ts
@@ -348,9 +348,9 @@ export type BaseRouter = {
   asPath: string
   basePath: string
   locale?: string | undefined
-  locales?: string[] | undefined
+  locales?: readonly string[] | undefined
   defaultLocale?: string | undefined
-  domainLocales?: DomainLocale[] | undefined
+  domainLocales?: readonly DomainLocale[] | undefined
   isLocaleDomain: boolean
 }
 
@@ -681,9 +681,9 @@ export default class Router implements BaseRouter {
   isSsr: boolean
   _inFlightRoute?: string | undefined
   _shallow?: boolean | undefined
-  locales?: string[] | undefined
+  locales?: readonly string[] | undefined
   defaultLocale?: string | undefined
-  domainLocales?: DomainLocale[] | undefined
+  domainLocales?: readonly DomainLocale[] | undefined
   isReady: boolean
   isLocaleDomain: boolean
   isFirstPopStateEvent = true
@@ -735,9 +735,9 @@ export default class Router implements BaseRouter {
       err?: Error
       isFallback: boolean
       locale?: string
-      locales?: string[]
+      locales?: readonly string[]
       defaultLocale?: string
-      domainLocales?: DomainLocale[]
+      domainLocales?: readonly DomainLocale[]
       isPreview?: boolean
     }
   ) {
diff --git a/packages/next/src/shared/lib/router/utils/get-next-pathname-info.ts b/packages/next/src/shared/lib/router/utils/get-next-pathname-info.ts
index 71937afa6db63..7016cfe47e801 100644
--- a/packages/next/src/shared/lib/router/utils/get-next-pathname-info.ts
+++ b/packages/next/src/shared/lib/router/utils/get-next-pathname-info.ts
@@ -39,7 +39,7 @@ interface Options {
    */
   nextConfig?: {
     basePath?: string
-    i18n?: { locales?: string[] } | null
+    i18n?: { locales?: readonly string[] } | null
     trailingSlash?: boolean
   }
 
diff --git a/packages/next/src/shared/lib/router/utils/resolve-rewrites.ts b/packages/next/src/shared/lib/router/utils/resolve-rewrites.ts
index 4c1bb545147b6..a628d474f223c 100644
--- a/packages/next/src/shared/lib/router/utils/resolve-rewrites.ts
+++ b/packages/next/src/shared/lib/router/utils/resolve-rewrites.ts
@@ -17,7 +17,7 @@ export default function resolveRewrites(
   },
   query: ParsedUrlQuery,
   resolveHref: (path: string) => string,
-  locales?: string[]
+  locales?: readonly string[]
 ): {
   matchedPage: boolean
   parsedAs: ParsedRelativeUrl
diff --git a/packages/next/src/shared/lib/utils.ts b/packages/next/src/shared/lib/utils.ts
index ff1955ab2456c..abc5d6da627f3 100644
--- a/packages/next/src/shared/lib/utils.ts
+++ b/packages/next/src/shared/lib/utils.ts
@@ -107,9 +107,9 @@ export type NEXT_DATA = {
   gip?: boolean
   appGip?: boolean
   locale?: string
-  locales?: string[]
+  locales?: readonly string[]
   defaultLocale?: string
-  domainLocales?: DomainLocale[]
+  domainLocales?: readonly DomainLocale[]
   scriptLoader?: any[]
   isPreview?: boolean
   notFoundSrcPage?: string
@@ -150,7 +150,7 @@ export interface NextPageContext {
   /**
    * All configured locales
    */
-  locales?: string[]
+  locales?: readonly string[]
   /**
    * The configured default locale
    */