diff --git a/packages/cli-server-api/src/statusPageMiddleware.ts b/packages/cli-server-api/src/statusPageMiddleware.ts
index b7d690752..2d99ed095 100644
--- a/packages/cli-server-api/src/statusPageMiddleware.ts
+++ b/packages/cli-server-api/src/statusPageMiddleware.ts
@@ -5,6 +5,7 @@
  * LICENSE file in the root directory of this source tree.
  */
 import http from 'http';
+import {normalizeProjectRoot} from '@react-native-community/cli-tools';
 
 /**
  * Status page so that anyone who needs to can verify that the packager is
@@ -16,7 +17,7 @@ export default function statusPageMiddleware(
 ) {
   res.setHeader(
     'X-React-Native-Project-Root',
-    new URL(`file:///${process.cwd()}`).pathname.slice(1),
+    normalizeProjectRoot(process.cwd()),
   );
   res.end('packager-status:running');
 }
diff --git a/packages/cli-tools/src/__tests__/findDevServerPort.test.ts b/packages/cli-tools/src/__tests__/findDevServerPort.test.ts
new file mode 100644
index 000000000..d130c02af
--- /dev/null
+++ b/packages/cli-tools/src/__tests__/findDevServerPort.test.ts
@@ -0,0 +1,69 @@
+import findDevServerPort from '../findDevServerPort';
+import isPackagerRunning from '../isPackagerRunning';
+import handlePortUnavailable from '../handlePortUnavailable';
+
+jest.mock('../isPackagerRunning');
+jest.mock('../handlePortUnavailable');
+jest.mock('../port', () => ({
+  logAlreadyRunningBundler: jest.fn(),
+  askForPortChange: jest.fn(),
+  logChangePortInstructions: jest.fn(),
+}));
+
+const isPackagerRunningMock = isPackagerRunning as jest.MockedFunction<
+  typeof isPackagerRunning
+>;
+const handlePortUnavailableMock = handlePortUnavailable as jest.MockedFunction<
+  typeof handlePortUnavailable
+>;
+
+beforeEach(() => {
+  jest.clearAllMocks();
+  handlePortUnavailableMock.mockResolvedValue({port: 8082, packager: true});
+});
+
+test('reuses the running dev server when the project root has a space', async () => {
+  const root = '/Users/me/my app';
+
+  isPackagerRunningMock.mockResolvedValue({
+    status: 'running',
+    root: '/Users/me/my%20app',
+  });
+
+  await expect(findDevServerPort(8081, root)).resolves.toEqual({
+    port: 8081,
+    startPackager: false,
+  });
+  expect(handlePortUnavailableMock).not.toHaveBeenCalled();
+});
+
+test('reuses the running dev server for a plain ASCII project root', async () => {
+  const root = '/Users/me/project';
+
+  isPackagerRunningMock.mockResolvedValue({
+    status: 'running',
+    root: '/Users/me/project',
+  });
+
+  await expect(findDevServerPort(8081, root)).resolves.toEqual({
+    port: 8081,
+    startPackager: false,
+  });
+  expect(handlePortUnavailableMock).not.toHaveBeenCalled();
+});
+
+test('asks for another port when a different project owns the port', async () => {
+  isPackagerRunningMock.mockResolvedValue({
+    status: 'running',
+    root: '/Users/me/other',
+  });
+
+  await expect(findDevServerPort(8081, '/Users/me/project')).resolves.toEqual({
+    port: 8082,
+    startPackager: true,
+  });
+  expect(handlePortUnavailableMock).toHaveBeenCalledWith(
+    8081,
+    '/Users/me/project',
+  );
+});
diff --git a/packages/cli-tools/src/__tests__/normalizeProjectRoot.test.ts b/packages/cli-tools/src/__tests__/normalizeProjectRoot.test.ts
new file mode 100644
index 000000000..4adb27b11
--- /dev/null
+++ b/packages/cli-tools/src/__tests__/normalizeProjectRoot.test.ts
@@ -0,0 +1,18 @@
+import normalizeProjectRoot from '../normalizeProjectRoot';
+
+test('leaves a plain posix path untouched', () => {
+  expect(normalizeProjectRoot('/Users/me/project')).toBe('/Users/me/project');
+});
+
+test('percent-encodes characters that are not valid in a header', () => {
+  expect(normalizeProjectRoot('/Users/me/my app')).toBe('/Users/me/my%20app');
+  expect(normalizeProjectRoot('/привіт/path')).toBe(
+    '/%D0%BF%D1%80%D0%B8%D0%B2%D1%96%D1%82/path',
+  );
+});
+
+test('converts a windows path to forward slashes', () => {
+  expect(normalizeProjectRoot('C:\\Users\\me\\project')).toBe(
+    'C:/Users/me/project',
+  );
+});
diff --git a/packages/cli-tools/src/findDevServerPort.ts b/packages/cli-tools/src/findDevServerPort.ts
index e3abfe945..0201ffd9d 100644
--- a/packages/cli-tools/src/findDevServerPort.ts
+++ b/packages/cli-tools/src/findDevServerPort.ts
@@ -1,5 +1,6 @@
 import handlePortUnavailable from './handlePortUnavailable';
 import isPackagerRunning from './isPackagerRunning';
+import normalizeProjectRoot from './normalizeProjectRoot';
 import {logAlreadyRunningBundler} from './port';
 
 const findDevServerPort = async (
@@ -18,7 +19,7 @@ const findDevServerPort = async (
     typeof packagerStatus === 'object' &&
     packagerStatus.status === 'running'
   ) {
-    if (packagerStatus.root === root) {
+    if (packagerStatus.root === normalizeProjectRoot(root)) {
       startPackager = false;
       logAlreadyRunningBundler(port);
     } else {
diff --git a/packages/cli-tools/src/getNextPort.ts b/packages/cli-tools/src/getNextPort.ts
index be4a5a4d0..26320d269 100644
--- a/packages/cli-tools/src/getNextPort.ts
+++ b/packages/cli-tools/src/getNextPort.ts
@@ -1,4 +1,5 @@
 import isPackagerRunning from './isPackagerRunning';
+import normalizeProjectRoot from './normalizeProjectRoot';
 
 type Result = {
   start: boolean;
@@ -19,10 +20,7 @@ const getNextPort = async (port: number, root: string): Promise<Result> => {
 
   const isRunning = typeof result === 'object' && result.status === 'running';
 
-  if (
-    isRunning &&
-    result.root === new URL(`file:///${root}`).pathname.slice(1)
-  ) {
+  if (isRunning && result.root === normalizeProjectRoot(root)) {
     // Found running bundler for this project, so we do not need to start packager!
     start = false;
   } else if (isRunning || result === 'unrecognized') {
diff --git a/packages/cli-tools/src/index.ts b/packages/cli-tools/src/index.ts
index 19132c2aa..7cefb9620 100644
--- a/packages/cli-tools/src/index.ts
+++ b/packages/cli-tools/src/index.ts
@@ -12,6 +12,7 @@ export * from './prompt';
 export * as link from './doclink';
 export {default as startServerInNewWindow} from './startServerInNewWindow';
 export {default as findDevServerPort} from './findDevServerPort';
+export {default as normalizeProjectRoot} from './normalizeProjectRoot';
 export * from './port';
 export {default as cacheManager} from './cacheManager';
 export {default as runSudo} from './runSudo';
diff --git a/packages/cli-tools/src/normalizeProjectRoot.ts b/packages/cli-tools/src/normalizeProjectRoot.ts
new file mode 100644
index 000000000..49813204d
--- /dev/null
+++ b/packages/cli-tools/src/normalizeProjectRoot.ts
@@ -0,0 +1,13 @@
+/**
+ * Normalizes a project root to the form used by the dev server's
+ * `X-React-Native-Project-Root` header.
+ *
+ * A header value has to be a valid ISO-8859-1 string, so the dev server
+ * percent-encodes the path before sending it. Anything comparing a local
+ * project root against that header has to apply the same transformation,
+ * otherwise roots containing characters that get encoded (a space, an accent,
+ * a non-latin script) never match.
+ */
+export default function normalizeProjectRoot(root: string): string {
+  return new URL(`file:///${root}`).pathname.slice(1);
+}