diff --git a/packages/playwright-core/src/client/fetch.ts b/packages/playwright-core/src/client/fetch.ts
index 970c22d11d..2f74bdfe2d 100644
--- a/packages/playwright-core/src/client/fetch.ts
+++ b/packages/playwright-core/src/client/fetch.ts
@@ -142,14 +142,18 @@ export class APIRequestContext extends ChannelOwner<channels.APIRequestContextCh
       let multipartData: channels.FormField[] | undefined;
       let postDataBuffer: Buffer | undefined;
       if (options.data !== undefined) {
-        if (isString(options.data))
-          postDataBuffer = Buffer.from(options.data, 'utf8');
-        else if (Buffer.isBuffer(options.data))
+        if (isString(options.data)) {
+          if (isJsonContentType(headers))
+            jsonData = options.data;
+          else
+            postDataBuffer = Buffer.from(options.data, 'utf8');
+        } else if (Buffer.isBuffer(options.data)) {
           postDataBuffer = options.data;
-        else if (typeof options.data === 'object')
+        } else if (typeof options.data === 'object' || typeof options.data === 'number' || typeof options.data === 'boolean') {
           jsonData = options.data;
-        else
+        } else {
           throw new Error(`Unexpected 'data' type`);
+        }
       } else if (options.form) {
         formData = objectToArray(options.form);
       } else if (options.multipart) {
@@ -300,4 +304,14 @@ async function readStreamToJson(stream: fs.ReadStream): Promise<ServerFilePayloa
     mimeType: mime.getType(streamPath) || 'application/octet-stream',
     buffer: buffer.toString('base64'),
   };
+}
+
+function isJsonContentType(headers?: HeadersArray): boolean {
+  if (!headers)
+    return false;
+  for (const { name, value } of headers) {
+    if (name.toLocaleLowerCase() === 'content-type')
+      return value === 'application/json';
+  }
+  return false;
 }
\ No newline at end of file
diff --git a/packages/playwright-core/src/server/fetch.ts b/packages/playwright-core/src/server/fetch.ts
index 2d1f51dafc..4484b9616b 100644
--- a/packages/playwright-core/src/server/fetch.ts
+++ b/packages/playwright-core/src/server/fetch.ts
@@ -488,10 +488,24 @@ function parseCookie(header: string): types.NetworkCookie | null {
   return cookie;
 }
 
+function isJsonParsable(value: any) {
+  if (typeof value !== 'string')
+    return false;
+  try {
+    JSON.parse(value);
+    return true;
+  } catch (e) {
+    if (e instanceof SyntaxError)
+      return false;
+    else
+      throw e;
+  }
+}
+
 function serializePostData(params: channels.APIRequestContextFetchParams, headers: { [name: string]: string }): Buffer | undefined {
   assert((params.postData ? 1 : 0) + (params.jsonData ? 1 : 0) + (params.formData ? 1 : 0) + (params.multipartData ? 1 : 0) <= 1, `Only one of 'data', 'form' or 'multipart' can be specified`);
   if (params.jsonData) {
-    const json = JSON.stringify(params.jsonData);
+    const json = isJsonParsable(params.jsonData) ? params.jsonData : JSON.stringify(params.jsonData);
     headers['content-type'] ??= 'application/json';
     return Buffer.from(json, 'utf8');
   } else if (params.formData) {
diff --git a/tests/global-fetch.spec.ts b/tests/global-fetch.spec.ts
index 794d80ddcb..002969fd77 100644
--- a/tests/global-fetch.spec.ts
+++ b/tests/global-fetch.spec.ts
@@ -234,3 +234,63 @@ it('should remove content-length from reidrected post requests', async ({ playwr
   expect(req2.headers['content-length']).toBe(undefined);
   await request.dispose();
 });
+
+
+const serialization = [
+  ['object', { 'foo': 'bar' }],
+  ['array', ['foo', 'bar', 2021]],
+  ['string', 'foo'],
+  ['bool', true],
+  ['number', 2021],
+];
+for (const [type, value] of serialization) {
+  const stringifiedValue = JSON.stringify(value);
+  it(`should json stringify ${type} body when content-type is application/json`, async ({ playwright, server }) => {
+    const request = await playwright.request.newContext();
+    const [req] = await Promise.all([
+      server.waitForRequest('/empty.html'),
+      request.post(server.EMPTY_PAGE, {
+        headers: {
+          'content-type': 'application/json'
+        },
+        data: value
+      })
+    ]);
+    const body = await req.postBody;
+    expect(body.toString()).toEqual(stringifiedValue);
+    await request.dispose();
+  });
+
+  it(`should not double stringify ${type} body when content-type is application/json`, async ({ playwright, server }) => {
+    const request = await playwright.request.newContext();
+    const [req] = await Promise.all([
+      server.waitForRequest('/empty.html'),
+      request.post(server.EMPTY_PAGE, {
+        headers: {
+          'content-type': 'application/json'
+        },
+        data: stringifiedValue
+      })
+    ]);
+    const body = await req.postBody;
+    expect(body.toString()).toEqual(stringifiedValue);
+    await request.dispose();
+  });
+}
+
+it(`should accept already serialized data as Buffer when content-type is application/json`, async ({ playwright, server }) => {
+  const request = await playwright.request.newContext();
+  const value = JSON.stringify(JSON.stringify({ 'foo': 'bar' }));
+  const [req] = await Promise.all([
+    server.waitForRequest('/empty.html'),
+    request.post(server.EMPTY_PAGE, {
+      headers: {
+        'content-type': 'application/json'
+      },
+      data: Buffer.from(value, 'utf8')
+    })
+  ]);
+  const body = await req.postBody;
+  expect(body.toString()).toEqual(value);
+  await request.dispose();
+});