diff --git a/packages/schematics/angular/migrations/update-9/update-app-tsconfigs.ts b/packages/schematics/angular/migrations/update-9/update-app-tsconfigs.ts
index 05f6d47e40e6..05d454f612ee 100644
--- a/packages/schematics/angular/migrations/update-9/update-app-tsconfigs.ts
+++ b/packages/schematics/angular/migrations/update-9/update-app-tsconfigs.ts
@@ -7,14 +7,20 @@
  */
 import { JsonAstObject, logging } from '@angular-devkit/core';
 import { Rule, Tree, UpdateRecorder } from '@angular-devkit/schematics';
-import { posix } from 'path';
+import { dirname, relative } from 'path';
 import {
   findPropertyInAstObject,
   insertPropertyInAstObjectInOrder,
   removePropertyInAstObject,
 } from '../../utility/json-utils';
 import { Builders } from '../../utility/workspace-models';
-import { getAllOptions, getTargets, getWorkspace, readJsonFileAsAstObject } from './utils';
+import {
+  forwardSlashPath,
+  getAllOptions,
+  getTargets,
+  getWorkspace,
+  readJsonFileAsAstObject,
+} from './utils';
 
 /**
  * Update the tsconfig files for applications
@@ -107,15 +113,18 @@ function updateTsConfig(tree: Tree, builderConfig: JsonAstObject, builderName: B
       const files = findPropertyInAstObject(tsConfigAst, 'files');
       if (!files) {
         const newFiles: string[] = [];
+        const tsConfigDir = dirname(forwardSlashPath(tsConfigPath));
 
         const mainOption = findPropertyInAstObject(option, 'main');
         if (mainOption && mainOption.kind === 'string') {
-          newFiles.push(posix.relative(posix.dirname(tsConfigPath), mainOption.value));
+          newFiles.push(
+            forwardSlashPath(relative(tsConfigDir, forwardSlashPath(mainOption.value))));
         }
 
         const polyfillsOption = findPropertyInAstObject(option, 'polyfills');
         if (polyfillsOption && polyfillsOption.kind === 'string') {
-          newFiles.push(posix.relative(posix.dirname(tsConfigPath), polyfillsOption.value));
+          newFiles.push(
+            forwardSlashPath(relative(tsConfigDir, forwardSlashPath(polyfillsOption.value))));
         }
 
         if (newFiles.length) {
diff --git a/packages/schematics/angular/migrations/update-9/update-app-tsconfigs_spec.ts b/packages/schematics/angular/migrations/update-9/update-app-tsconfigs_spec.ts
index 2459b8c6f218..840b7c3492ca 100644
--- a/packages/schematics/angular/migrations/update-9/update-app-tsconfigs_spec.ts
+++ b/packages/schematics/angular/migrations/update-9/update-app-tsconfigs_spec.ts
@@ -8,6 +8,7 @@
 
 import { EmptyTree } from '@angular-devkit/schematics';
 import { SchematicTestRunner, UnitTestTree } from '@angular-devkit/schematics/testing';
+import { getWorkspaceTargets, updateWorkspaceTargets } from './update-workspace-config_spec';
 
 // tslint:disable-next-line: no-any
 function overrideJsonFile(tree: UnitTestTree, path: string, newContent: object) {
@@ -64,6 +65,32 @@ describe('Migration to version 9', () => {
       expect(files).toEqual(['src/main.ts', 'src/polyfills.ts']);
     });
 
+    it('should resolve paths correctly even if they are using windows separators', async () => {
+      const tree2 = await schematicRunner
+        .runExternalSchematicAsync(
+          require.resolve('../../collection.json'),
+          'application',
+          {
+            name: 'another-app',
+          },
+          tree,
+        )
+        .toPromise();
+
+      const tsCfgPath = 'projects/another-app/tsconfig.app.json';
+      overrideJsonFile(tree2, tsCfgPath, defaultTsConfigOptions);
+      const config = getWorkspaceTargets(tree2, 'another-app');
+      config.build.options.main = 'projects\\another-app\\src\\main.ts';
+      config.build.options.polyfills = 'projects\\another-app\\src\\polyfills.ts';
+      config.build.options.tsConfig = 'projects\\another-app\\tsconfig.app.json';
+      updateWorkspaceTargets(tree2, config, 'another-app');
+
+      const tree3 = await schematicRunner.runSchematicAsync('workspace-version-9', {}, tree2.branch()).toPromise();
+      const { exclude, files } = JSON.parse(tree3.readContent(tsCfgPath));
+      expect(exclude).toBeUndefined();
+      expect(files).toEqual(['src/main.ts', 'src/polyfills.ts']);
+    });
+
     it('should update apps tsConfig when tsconfig has include', async () => {
       const tsConfigContent = {
         ...defaultTsConfigOptions,
diff --git a/packages/schematics/angular/migrations/update-9/update-workspace-config_spec.ts b/packages/schematics/angular/migrations/update-9/update-workspace-config_spec.ts
index 39a10eeffebc..54533ff71138 100644
--- a/packages/schematics/angular/migrations/update-9/update-workspace-config_spec.ts
+++ b/packages/schematics/angular/migrations/update-9/update-workspace-config_spec.ts
@@ -13,14 +13,17 @@ import { WorkspaceTargets } from '../../utility/workspace-models';
 import { ANY_COMPONENT_STYLE_BUDGET } from './update-workspace-config';
 
 // tslint:disable-next-line: no-any
-function getWorkspaceTargets(tree: UnitTestTree): any {
-  return JSON.parse(tree.readContent(workspacePath))
-    .projects['migration-test'].architect;
+export function getWorkspaceTargets(tree: UnitTestTree, project = 'migration-test'): any {
+  return JSON.parse(tree.readContent(workspacePath)).projects[project].architect;
 }
 
-function updateWorkspaceTargets(tree: UnitTestTree, workspaceTargets: WorkspaceTargets) {
+export function updateWorkspaceTargets(
+  tree: UnitTestTree,
+  workspaceTargets: WorkspaceTargets,
+  project = 'migration-test',
+) {
   const config = JSON.parse(tree.readContent(workspacePath));
-  config.projects['migration-test'].architect = workspaceTargets;
+  config.projects[project].architect = workspaceTargets;
   tree.overwrite(workspacePath, JSON.stringify(config, undefined, 2));
 }
 
diff --git a/packages/schematics/angular/migrations/update-9/utils.ts b/packages/schematics/angular/migrations/update-9/utils.ts
index f80a0dd290d1..8cb4261ac022 100644
--- a/packages/schematics/angular/migrations/update-9/utils.ts
+++ b/packages/schematics/angular/migrations/update-9/utils.ts
@@ -144,3 +144,9 @@ export function isIvyEnabled(tree: Tree, tsConfigPath: string): boolean {
 
   return true;
 }
+
+// TS represents paths internally with '/' and expects paths to be in this format.
+// angular.json expects paths with '/', but doesn't enforce them.
+export function forwardSlashPath(path: string) {
+  return path.replace(/\\/g, '/');
+}