diff --git a/packages/angular_devkit/build_angular/src/builders/jest/index.ts b/packages/angular_devkit/build_angular/src/builders/jest/index.ts
index 64491c8bcb04..250cdb26c2fd 100644
--- a/packages/angular_devkit/build_angular/src/builders/jest/index.ts
+++ b/packages/angular_devkit/build_angular/src/builders/jest/index.ts
@@ -8,6 +8,8 @@
 
 import { BuilderContext, BuilderOutput, createBuilder } from '@angular-devkit/architect';
 import { execFile as execFileCb } from 'child_process';
+import { promises as fs } from 'fs';
+import { platform } from 'os';
 import * as path from 'path';
 import { promisify } from 'util';
 import { colors } from '../../utils/color';
@@ -20,6 +22,8 @@ import { Schema as JestBuilderSchema } from './schema';
 
 const execFile = promisify(execFileCb);
 
+const USING_WINDOWS = platform() === 'win32';
+
 /** Main execution function for the Jest builder. */
 export default createBuilder(
   async (schema: JestBuilderSchema, context: BuilderContext): Promise<BuilderOutput> => {
@@ -54,8 +58,25 @@ export default createBuilder(
       };
     }
 
+    const [testFiles, customConfig] = await Promise.all([
+      findTestFiles(options.include, options.exclude, context.workspaceRoot),
+      findCustomJestConfig(context.workspaceRoot),
+    ]);
+
+    // Warn if a custom Jest configuration is found. We won't use it, so if a developer is trying to use a custom config, this hopefully
+    // makes a better experience than silently ignoring the configuration.
+    // Ideally, this would be a hard error. However a Jest config could exist for testing other files in the workspace outside of Angular
+    // CLI, so we likely can't produce a hard error in this situation without an opt-out.
+    if (customConfig) {
+      context.logger.warn(
+        'A custom Jest config was found, but this is not supported by `@angular-devkit/build-angular:jest` and will be' +
+          ` ignored: ${customConfig}. This is an experiment to see if completely abstracting away Jest's configuration is viable. Please` +
+          ` consider if your use case can be met without directly modifying the Jest config. If this is a major obstacle for your use` +
+          ` case, please post it in this issue so we can collect feedback and evaluate: https://github.com/angular/angular-cli/issues/25434.`,
+      );
+    }
+
     // Build all the test files.
-    const testFiles = await findTestFiles(options.include, options.exclude, context.workspaceRoot);
     const jestGlobal = path.join(__dirname, 'jest-global.mjs');
     const initTestBed = path.join(__dirname, 'init-test-bed.mjs');
     const buildResult = await build(context, {
@@ -85,6 +106,7 @@ export default createBuilder(
       jest,
 
       `--rootDir="${path.join(testOut, 'browser')}"`,
+      `--config=${path.join(__dirname, 'jest.config.mjs')}`,
       '--testEnvironment=jsdom',
 
       // TODO(dgp1130): Enable cache once we have a mechanism for properly clearing / disabling it.
@@ -162,3 +184,17 @@ function resolveModule(module: string): string | undefined {
     return undefined;
   }
 }
+
+/** Returns whether or not the provided directory includes a Jest configuration file. */
+async function findCustomJestConfig(dir: string): Promise<string | undefined> {
+  const entries = await fs.readdir(dir, { withFileTypes: true });
+
+  // Jest supports many file extensions (`js`, `ts`, `cjs`, `cts`, `json`, etc.) Just look
+  // for anything with that prefix.
+  const config = entries.find((entry) => entry.isFile() && entry.name.startsWith('jest.config.'));
+  if (!config) {
+    return undefined;
+  }
+
+  return path.join(dir, config.name);
+}
diff --git a/packages/angular_devkit/build_angular/src/builders/jest/jest.config.mjs b/packages/angular_devkit/build_angular/src/builders/jest/jest.config.mjs
new file mode 100644
index 000000000000..3fa377339eda
--- /dev/null
+++ b/packages/angular_devkit/build_angular/src/builders/jest/jest.config.mjs
@@ -0,0 +1,11 @@
+/**
+ * @license
+ * Copyright Google LLC All Rights Reserved.
+ *
+ * Use of this source code is governed by an MIT-style license that can be
+ * found in the LICENSE file at https://angular.io/license
+ */
+
+// Empty config file, everything is specified via CLI options right now.
+// This file is used just so Jest doesn't accidentally inherit a custom user-specified Jest config.
+export default {};
diff --git a/tests/legacy-cli/e2e/tests/jest/custom-config.ts b/tests/legacy-cli/e2e/tests/jest/custom-config.ts
new file mode 100644
index 000000000000..16f4d5284ee2
--- /dev/null
+++ b/tests/legacy-cli/e2e/tests/jest/custom-config.ts
@@ -0,0 +1,25 @@
+import { writeFile } from '../../utils/fs';
+import { applyJestBuilder } from '../../utils/jest';
+import { ng } from '../../utils/process';
+
+export default async function (): Promise<void> {
+  await applyJestBuilder();
+
+  // Users may incorrectly write a Jest config believing it to be used by Angular.
+  await writeFile(
+    'jest.config.mjs',
+    `
+export default {
+  runner: 'does-not-exist',
+};
+  `.trim(),
+  );
+
+  // Should not fail from the above (broken) configuration. Shouldn't use it at all.
+  const { stderr } = await ng('test');
+
+  // Should warn that a Jest configuration was found but not used.
+  if (!stderr.includes('A custom Jest config was found')) {
+    throw new Error(`No warning about custom Jest config:\nSTDERR:\n\n${stderr}`);
+  }
+}