diff --git a/packages/vite/src/node/ssr/ssrExternal.ts b/packages/vite/src/node/ssr/ssrExternal.ts
index 5c04e78ac067eb..d81634a1a485ca 100644
--- a/packages/vite/src/node/ssr/ssrExternal.ts
+++ b/packages/vite/src/node/ssr/ssrExternal.ts
@@ -1,7 +1,14 @@
+import fs from 'fs'
 import path from 'path'
 import type { InternalResolveOptions } from '../plugins/resolve'
 import { tryNodeResolve } from '../plugins/resolve'
-import { createDebugger, isDefined, lookupFile, resolveFrom } from '../utils'
+import {
+  createDebugger,
+  isDefined,
+  lookupFile,
+  normalizePath,
+  resolveFrom
+} from '../utils'
 import type { Logger, ResolvedConfig } from '..'
 import { createFilter } from '@rollup/pluginutils'
 
@@ -105,6 +112,7 @@ function collectExternals(
     seen.add(id)
 
     let esmEntry: string | undefined
+    let requireEntry: string
     try {
       esmEntry = tryNodeResolve(
         id,
@@ -114,6 +122,9 @@ function collectExternals(
         undefined,
         true
       )?.id
+      // normalizePath required for windows. tryNodeResolve uses normalizePath
+      // which returns with '/', require.resolve returns with '\\'
+      requireEntry = normalizePath(require.resolve(id, { paths: [root] }))
     } catch (e) {
       try {
         // no main entry, but deep imports may be allowed
@@ -130,6 +141,29 @@ function collectExternals(
       debug(`Failed to resolve entries for package "${id}"\n`, e)
       continue
     }
+
+    const pkgPath = resolveFrom(`${id}/package.json`, root)
+    const pkgContent = fs.readFileSync(pkgPath, 'utf-8')
+
+    if (!pkgContent) {
+      continue
+    }
+    const pkg = JSON.parse(pkgContent)
+
+    if (pkg.type === 'module') {
+      if (requireEntry && !requireEntry.endsWith('.cjs')) {
+        logger.warn(
+          `${id} must use a .cjs extension for the CJS entry point when "type": "module" is set. Please contact the package author to fix.`
+        )
+      }
+    } else {
+      if (esmEntry && !esmEntry.endsWith('.mjs')) {
+        logger.warn(
+          `${id} must either set "type": "module" or use an .mjs extension for the ESM entry point. Please contact the package author to fix.`
+        )
+      }
+    }
+
     if (esmEntry && !esmEntry.includes('node_modules')) {
       // trace the dependencies of linked packages
       const pkgPath = resolveFrom(`${id}/package.json`, root)