diff --git a/packages/compiler-cli/src/transformers/program.ts b/packages/compiler-cli/src/transformers/program.ts
index 79b3ee4f18..8a10d93529 100644
--- a/packages/compiler-cli/src/transformers/program.ts
+++ b/packages/compiler-cli/src/transformers/program.ts
@@ -974,10 +974,8 @@ function normalizeSeparators(path: string): string {
  * TODO(tbosch): talk to the TypeScript team to expose their logic for calculating the `rootDir`
  * if none was specified.
  *
- * Note: This function works on normalized paths from typescript.
- *
- * @param outDir
- * @param outSrcMappings
+ * Note: This function works on normalized paths from typescript but should always return
+ * POSIX normalized paths for output paths.
  */
 export function createSrcToOutPathMapper(
     outDir: string | undefined, sampleSrcFileName: string | undefined,
@@ -986,7 +984,6 @@ export function createSrcToOutPathMapper(
       resolve: typeof path.resolve,
       relative: typeof path.relative
     } = path): (srcFileName: string) => string {
-  let srcToOutPath: (srcFileName: string) => string;
   if (outDir) {
     let path: {} = {};  // Ensure we error if we use `path` instead of `host`.
     if (sampleSrcFileName == null || sampleOutFileName == null) {
@@ -1006,11 +1003,18 @@ export function createSrcToOutPathMapper(
            srcDirParts[srcDirParts.length - 1 - i] === outDirParts[outDirParts.length - 1 - i])
       i++;
     const rootDir = srcDirParts.slice(0, srcDirParts.length - i).join('/');
-    srcToOutPath = (srcFileName) => host.resolve(outDir, host.relative(rootDir, srcFileName));
+    return (srcFileName) => {
+      // Note: Before we return the mapped output path, we need to normalize the path delimiters
+      // because the output path is usually passed to TypeScript which sometimes only expects
+      // posix normalized paths (e.g. if a custom compiler host is used)
+      return normalizeSeparators(host.resolve(outDir, host.relative(rootDir, srcFileName)));
+    };
   } else {
-    srcToOutPath = (srcFileName) => srcFileName;
+    // Note: Before we return the output path, we need to normalize the path delimiters because
+    // the output path is usually passed to TypeScript which only passes around posix
+    // normalized paths (e.g. if a custom compiler host is used)
+    return (srcFileName) => normalizeSeparators(srcFileName);
   }
-  return srcToOutPath;
 }
 
 export function i18nExtract(
diff --git a/packages/compiler-cli/test/transformers/program_spec.ts b/packages/compiler-cli/test/transformers/program_spec.ts
index c5afa87a68..951b1ffbd1 100644
--- a/packages/compiler-cli/test/transformers/program_spec.ts
+++ b/packages/compiler-cli/test/transformers/program_spec.ts
@@ -604,13 +604,13 @@ describe('ng program', () => {
     it('should work on windows with normalized paths', () => {
       const mapper =
           createSrcToOutPathMapper('c:/tmp/out', 'c:/tmp/a/x.ts', 'c:/tmp/out/a/x.js', path.win32);
-      expect(mapper('c:/tmp/b/y.js')).toBe('c:\\tmp\\out\\b\\y.js');
+      expect(mapper('c:/tmp/b/y.js')).toBe('c:/tmp/out/b/y.js');
     });
 
     it('should work on windows with non-normalized paths', () => {
       const mapper = createSrcToOutPathMapper(
           'c:\\tmp\\out', 'c:\\tmp\\a\\x.ts', 'c:\\tmp\\out\\a\\x.js', path.win32);
-      expect(mapper('c:\\tmp\\b\\y.js')).toBe('c:\\tmp\\out\\b\\y.js');
+      expect(mapper('c:\\tmp\\b\\y.js')).toBe('c:/tmp/out/b/y.js');
     });
   });