diff --git a/packages/runtime/src/helpers/files.ts b/packages/runtime/src/helpers/files.ts
index 4da26852ef..d97d614fb8 100644
--- a/packages/runtime/src/helpers/files.ts
+++ b/packages/runtime/src/helpers/files.ts
@@ -18,7 +18,6 @@ import { Rewrites, RoutesManifest } from './types'
 import { findModuleFromBase } from './utils'
 
 const TEST_ROUTE = /(|\/)\[[^/]+?](\/|\.html|$)/
-const SOURCE_FILE_EXTENSIONS = ['js', 'jsx', 'ts', 'tsx']
 
 export const isDynamicRoute = (route) => TEST_ROUTE.test(route)
 
@@ -341,12 +340,16 @@ const getServerFile = (root: string, includeBase = true) => {
   return findModuleFromBase({ candidates, paths: [root] })
 }
 
+// Next.js already defines a default `pageExtensions` array in its `required-server-files.json` file
+// In case it gets `undefined`, this is a fallback
+const SOURCE_FILE_EXTENSIONS = ['js', 'jsx', 'ts', 'tsx']
+
 /**
  * Find the source file for a given page route
  */
-export const getSourceFileForPage = (page: string, roots: string[]) => {
+export const getSourceFileForPage = (page: string, roots: string[], pageExtensions = SOURCE_FILE_EXTENSIONS) => {
   for (const root of roots) {
-    for (const extension of SOURCE_FILE_EXTENSIONS) {
+    for (const extension of pageExtensions) {
       const file = join(root, `${page}.${extension}`)
       if (existsSync(file)) {
         return file
diff --git a/packages/runtime/src/helpers/functions.ts b/packages/runtime/src/helpers/functions.ts
index 5d0f8032b7..aa2423922e 100644
--- a/packages/runtime/src/helpers/functions.ts
+++ b/packages/runtime/src/helpers/functions.ts
@@ -199,7 +199,11 @@ export const setupImageFunction = async ({
 /**
  * Look for API routes, and extract the config from the source file.
  */
-export const getApiRouteConfigs = async (publish: string, baseDir: string): Promise<Array<ApiRouteConfig>> => {
+export const getApiRouteConfigs = async (
+  publish: string,
+  baseDir: string,
+  pageExtensions: string[],
+): Promise<Array<ApiRouteConfig>> => {
   const pages = await readJSON(join(publish, 'server', 'pages-manifest.json'))
   const apiRoutes = Object.keys(pages).filter((page) => page.startsWith('/api/'))
   // two possible places
@@ -209,7 +213,7 @@ export const getApiRouteConfigs = async (publish: string, baseDir: string): Prom
 
   return await Promise.all(
     apiRoutes.map(async (apiRoute) => {
-      const filePath = getSourceFileForPage(apiRoute, [pagesDir, srcPagesDir])
+      const filePath = getSourceFileForPage(apiRoute, [pagesDir, srcPagesDir], pageExtensions)
       return { route: apiRoute, config: await extractConfigFromFile(filePath), compiled: pages[apiRoute] }
     }),
   )
@@ -218,8 +222,12 @@ export const getApiRouteConfigs = async (publish: string, baseDir: string): Prom
 /**
  * Looks for extended API routes (background and scheduled functions) and extract the config from the source file.
  */
-export const getExtendedApiRouteConfigs = async (publish: string, baseDir: string): Promise<Array<ApiRouteConfig>> => {
-  const settledApiRoutes = await getApiRouteConfigs(publish, baseDir)
+export const getExtendedApiRouteConfigs = async (
+  publish: string,
+  baseDir: string,
+  pageExtensions: string[],
+): Promise<Array<ApiRouteConfig>> => {
+  const settledApiRoutes = await getApiRouteConfigs(publish, baseDir, pageExtensions)
 
   // We only want to return the API routes that are background or scheduled functions
   return settledApiRoutes.filter((apiRoute) => apiRoute.config.type !== undefined)
diff --git a/packages/runtime/src/index.ts b/packages/runtime/src/index.ts
index 6fabcb5e0d..58a3e3604e 100644
--- a/packages/runtime/src/index.ts
+++ b/packages/runtime/src/index.ts
@@ -65,6 +65,7 @@ const plugin: NetlifyPlugin = {
     netlifyConfig.build.environment.NEXT_PRIVATE_TARGET = 'server'
   },
 
+  // eslint-disable-next-line max-lines-per-function
   async onBuild({
     constants,
     netlifyConfig,
@@ -79,11 +80,22 @@ const plugin: NetlifyPlugin = {
 
     checkNextSiteHasBuilt({ publish, failBuild })
 
-    const { appDir, basePath, i18n, images, target, ignore, trailingSlash, outdir, experimental, routesManifest } =
-      await getNextConfig({
-        publish,
-        failBuild,
-      })
+    const {
+      appDir,
+      basePath,
+      i18n,
+      images,
+      target,
+      ignore,
+      trailingSlash,
+      outdir,
+      experimental,
+      routesManifest,
+      pageExtensions,
+    } = await getNextConfig({
+      publish,
+      failBuild,
+    })
     await cleanupEdgeFunctions(constants)
 
     const middlewareManifest = await loadMiddlewareManifest(netlifyConfig)
@@ -150,7 +162,7 @@ const plugin: NetlifyPlugin = {
     const buildId = readFileSync(join(publish, 'BUILD_ID'), 'utf8').trim()
 
     await configureHandlerFunctions({ netlifyConfig, ignore, publish: relative(process.cwd(), publish) })
-    const apiRoutes = await getExtendedApiRouteConfigs(publish, appDir)
+    const apiRoutes = await getExtendedApiRouteConfigs(publish, appDir, pageExtensions)
 
     await generateFunctions(constants, appDir, apiRoutes)
     await generatePagesResolver(constants)
diff --git a/test/fixtures/page-extensions/custom/pages/api/custom.api.js b/test/fixtures/page-extensions/custom/pages/api/custom.api.js
new file mode 100644
index 0000000000..625c0891b2
--- /dev/null
+++ b/test/fixtures/page-extensions/custom/pages/api/custom.api.js
@@ -0,0 +1 @@
+// noop
\ No newline at end of file
diff --git a/test/fixtures/page-extensions/default/pages/api/default.js b/test/fixtures/page-extensions/default/pages/api/default.js
new file mode 100644
index 0000000000..625c0891b2
--- /dev/null
+++ b/test/fixtures/page-extensions/default/pages/api/default.js
@@ -0,0 +1 @@
+// noop
\ No newline at end of file
diff --git a/test/helpers/files.spec.ts b/test/helpers/files.spec.ts
index 4e1cb3f596..02e77e3e5b 100644
--- a/test/helpers/files.spec.ts
+++ b/test/helpers/files.spec.ts
@@ -6,6 +6,7 @@ import {
   patchNextFiles,
   unpatchNextFiles,
   getDependenciesOfFile,
+  getSourceFileForPage,
 } from "../../packages/runtime/src/helpers/files"
 import {
   readFileSync,
@@ -19,6 +20,8 @@ import { join } from "pathe"
 import { Rewrites } from "../../packages/runtime/src/helpers/types"
 import { describeCwdTmpDir, moveNextDist } from "../test-utils"
 
+const TEST_DIR = resolve(__dirname, '..')
+
 const REDIRECTS: Rewrites = [
   {
     source: '/:file((?!\\.well-known(?:/.*)?)(?:[^/]+/)*[^/]+\\.\\w+)/',
@@ -215,7 +218,27 @@ describe('dependency tracing', () => {
   it('generates dependency list from a source file', async () => {
     const dependencies = await getDependenciesOfFile(resolve(__dirname, '../fixtures/analysis/background.js'))
     expect(dependencies).toEqual(
-      ['test/webpack-api-runtime.js', 'package.json'].map((dep) => resolve(dirname(resolve(__dirname, '..')), dep)),
+      ['test/webpack-api-runtime.js', 'package.json'].map((dep) => resolve(dirname(TEST_DIR), dep)),
     )
   })
+})
+
+describe('getSourceFileForPage', () => {
+  it('handles default pageExtensions', () => {
+    const pagesDir = resolve(__dirname, '../fixtures/page-extensions/default/pages')
+    const apiRoute = '/api/default'
+
+    const filePath = getSourceFileForPage(apiRoute, [pagesDir])
+
+    expect(filePath.replace(TEST_DIR, '')).toBe('/fixtures/page-extensions/default/pages/api/default.js')
+  })
+
+  it('handles custom pageExtensions', () => {
+    const pagesDir = resolve(__dirname, '../fixtures/page-extensions/custom/pages')
+    const apiRoute = '/api/custom'
+
+    const filePath = getSourceFileForPage(apiRoute, [pagesDir], ['api.js'])
+
+    expect(filePath.replace(TEST_DIR, '')).toBe('/fixtures/page-extensions/custom/pages/api/custom.api.js')
+  })
 })
\ No newline at end of file