diff --git a/src/adapter/objectPreview/index.ts b/src/adapter/objectPreview/index.ts
index bdff413ef..89e245e49 100644
--- a/src/adapter/objectPreview/index.ts
+++ b/src/adapter/objectPreview/index.ts
@@ -600,6 +600,84 @@ export function formatAsTable(param: Cdp.Runtime.ObjectPreview): string {
   return table.map(row => stringUtils.trimEnd(row, maxTableWidth)).join('\n');
 }
 
+function formatPropertyPreviewAsJson(prop: Cdp.Runtime.PropertyPreview): string | undefined {
+  switch (prop.type) {
+    case 'string':
+      return JSON.stringify(prop.value ?? '');
+    case 'number': {
+      const num = Number(prop.value);
+      return isFinite(num) ? String(num) : 'null';
+    }
+    case 'boolean':
+      return prop.value === 'true' ? 'true' : 'false';
+    case 'undefined':
+    case 'function':
+    case 'symbol':
+    case 'accessor':
+      return undefined; // omit from JSON (matches JSON.stringify behavior)
+    case 'bigint':
+      return undefined; // not JSON serializable
+    case 'object':
+      if (prop.subtype === 'null') return 'null';
+      if (prop.valuePreview) return formatObjectPreviewAsJson(prop.valuePreview);
+      return '{}';
+    default:
+      return prop.value !== undefined ? JSON.stringify(prop.value) : 'null';
+  }
+}
+
+function formatObjectPreviewAsJson(preview: Cdp.Runtime.ObjectPreview): string {
+  if (preview.subtype === 'array' || preview.subtype === 'typedarray') {
+    const items: (string | undefined)[] = [];
+    for (const prop of preview.properties || []) {
+      if (!/^\d+$/.test(prop.name)) continue;
+      items[parseInt(prop.name, 10)] = formatPropertyPreviewAsJson(prop) ?? 'null';
+    }
+    const result: string[] = [];
+    for (let i = 0; i < items.length; i++) {
+      result.push(items[i] ?? 'null');
+    }
+    return '[' + result.join(',') + ']';
+  }
+
+  const parts: string[] = [];
+  for (const prop of preview.properties || []) {
+    const value = formatPropertyPreviewAsJson(prop);
+    if (value !== undefined) {
+      parts.push(JSON.stringify(prop.name) + ':' + value);
+    }
+  }
+  return '{' + parts.join(',') + '}';
+}
+
+function formatAsJson(param: ObjectPreview.AnyObject): string {
+  const raw = param as Cdp.Runtime.RemoteObject;
+
+  if (raw.type === 'string') {
+    return JSON.stringify(raw.value ?? raw.description ?? '');
+  }
+  if (raw.type === 'number') {
+    if (raw.unserializableValue) return 'null'; // NaN, Infinity, -Infinity → null in JSON
+    return String(raw.value ?? raw.description);
+  }
+  if (raw.type === 'boolean') {
+    return String(raw.value);
+  }
+  if (raw.type === 'undefined') {
+    return 'undefined';
+  }
+  if (raw.type === 'bigint' || raw.type === 'symbol') {
+    return 'undefined'; // not JSON serializable
+  }
+  if (raw.type === 'function') {
+    return 'undefined';
+  }
+  // raw.type === 'object'
+  if (raw.subtype === 'null') return 'null';
+  if (raw.preview) return formatObjectPreviewAsJson(raw.preview);
+  return '{}';
+}
+
 export const messageFormatters: messageFormat.Formatters<ObjectPreview.AnyObject> = new Map([
   ['', (param, context) => previewRemoteObjectInternal(param, context)],
   ['s', (param, context) => formatAsString(param as ObjectPreview.StringObj, context.budget)],
@@ -619,6 +697,7 @@ export const messageFormatters: messageFormat.Formatters<ObjectPreview.AnyObject
       formatAsNumber(param as ObjectPreview.Numeric, false, context.budget, undefined),
   ],
   ['c', param => messageFormat.formatCssAsAnsi((param as { value: string }).value)],
+  ['j', param => formatAsJson(param)],
   ['o', (param, context) => previewRemoteObjectInternal(param, context)],
   ['O', (param, context) => previewRemoteObjectInternal(param, context)],
 ]);
diff --git a/src/adapter/objectPreview/objectPreview.test.ts b/src/adapter/objectPreview/objectPreview.test.ts
new file mode 100644
index 000000000..739433a27
--- /dev/null
+++ b/src/adapter/objectPreview/objectPreview.test.ts
@@ -0,0 +1,172 @@
+/*---------------------------------------------------------
+ * Copyright (C) Microsoft Corporation. All rights reserved.
+ *--------------------------------------------------------*/
+
+import { expect } from 'chai';
+import { formatMessage } from '../messageFormat';
+import { messageFormatters } from './index';
+
+describe('objectPreview', () => {
+  describe('%j format specifier', () => {
+    const format = (fmt: string, ...args: object[]) =>
+      formatMessage(fmt, args as never, messageFormatters).result;
+
+    it('formats a simple object as JSON', () => {
+      expect(
+        format('%j', {
+          type: 'object',
+          subtype: undefined,
+          className: 'Object',
+          description: 'Object',
+          preview: {
+            type: 'object',
+            description: 'Object',
+            overflow: false,
+            properties: [{ name: 'host', type: 'string', value: 'localhost' }],
+          },
+        }),
+      ).to.equal('{"host":"localhost"}');
+    });
+
+    it('formats a nested object as JSON', () => {
+      expect(
+        format('%j', {
+          type: 'object',
+          subtype: undefined,
+          className: 'Object',
+          description: 'Object',
+          preview: {
+            type: 'object',
+            description: 'Object',
+            overflow: false,
+            properties: [
+              {
+                name: 'a',
+                type: 'object',
+                subtype: undefined,
+                valuePreview: {
+                  type: 'object',
+                  description: 'Object',
+                  overflow: false,
+                  properties: [{ name: 'b', type: 'number', value: '1' }],
+                },
+              },
+            ],
+          },
+        }),
+      ).to.equal('{"a":{"b":1}}');
+    });
+
+    it('formats an array as JSON', () => {
+      expect(
+        format('%j', {
+          type: 'object',
+          subtype: 'array',
+          className: 'Array',
+          description: 'Array(3)',
+          preview: {
+            type: 'object',
+            subtype: 'array',
+            description: 'Array(3)',
+            overflow: false,
+            properties: [
+              { name: '0', type: 'number', value: '1' },
+              { name: '1', type: 'number', value: '2' },
+              { name: '2', type: 'number', value: '3' },
+            ],
+          },
+        }),
+      ).to.equal('[1,2,3]');
+    });
+
+    it('formats null as JSON', () => {
+      expect(format('%j', { type: 'object', subtype: 'null' })).to.equal('null');
+    });
+
+    it('formats a string as JSON', () => {
+      expect(
+        format('%j', { type: 'string', value: 'hello', subtype: undefined }),
+      ).to.equal('"hello"');
+    });
+
+    it('formats a number as JSON', () => {
+      expect(
+        format('%j', { type: 'number', value: 42, description: '42', subtype: undefined }),
+      ).to.equal('42');
+    });
+
+    it('formats NaN as null in JSON', () => {
+      expect(
+        format('%j', {
+          type: 'number',
+          unserializableValue: 'NaN',
+          description: 'NaN',
+          subtype: undefined,
+        }),
+      ).to.equal('null');
+    });
+
+    it('formats a boolean as JSON', () => {
+      expect(
+        format('%j', { type: 'boolean', value: true, description: 'true', subtype: undefined }),
+      ).to.equal('true');
+    });
+
+    it('formats undefined as the string "undefined"', () => {
+      expect(format('%j', { type: 'undefined', subtype: undefined })).to.equal('undefined');
+    });
+
+    it('formats a function as the string "undefined"', () => {
+      expect(
+        format('%j', {
+          type: 'function',
+          subtype: undefined,
+          description: 'function() {}',
+        }),
+      ).to.equal('undefined');
+    });
+
+    it('handles mixed specifiers correctly', () => {
+      expect(
+        format(
+          '%s: %s %j',
+          { type: 'string', value: 'id', subtype: undefined },
+          { type: 'string', value: 'Request headers', subtype: undefined },
+          {
+            type: 'object',
+            subtype: undefined,
+            className: 'Object',
+            description: 'Object',
+            preview: {
+              type: 'object',
+              description: 'Object',
+              overflow: false,
+              properties: [{ name: 'host', type: 'string', value: 'localhost' }],
+            },
+          },
+        ),
+      ).to.equal('id: Request headers {"host":"localhost"}');
+    });
+
+    it('omits undefined properties from JSON object', () => {
+      expect(
+        format('%j', {
+          type: 'object',
+          subtype: undefined,
+          className: 'Object',
+          description: 'Object',
+          preview: {
+            type: 'object',
+            description: 'Object',
+            overflow: false,
+            properties: [
+              { name: 'a', type: 'string', value: 'hello' },
+              { name: 'fn', type: 'function', value: 'function()' },
+              { name: 'undef', type: 'undefined' },
+            ],
+          },
+        }),
+      ).to.equal('{"a":"hello"}');
+    });
+  });
+});
diff --git a/src/test/console/console-format-json-format.txt b/src/test/console/console-format-json-format.txt
new file mode 100644
index 000000000..231b202f4
--- /dev/null
+++ b/src/test/console/console-format-json-format.txt
@@ -0,0 +1,18 @@
+Evaluating: 'console.log("%j", {host: "localhost"})'
+stdout> {"host":"localhost"}
+stdout> > {"host":"localhost"}
+stdout>     > arg1: {host: 'localhost'}
+
+Evaluating: 'console.log("%s: %s %j", "id", "Request headers", {host: "localhost"})'
+stdout> id: Request headers {"host":"localhost"}
+stdout> > id: Request headers {"host":"localhost"}
+stdout>     > arg3: {host: 'localhost'}
+
+Evaluating: 'console.log("%j", [1, 2, 3])'
+stdout> [1,2,3]
+stdout> > [1,2,3]
+stdout>     > arg1: (3) [1, 2, 3]
+
+Evaluating: 'console.log("%j", null)'
+stdout> null
+
diff --git a/src/test/console/consoleFormatTest.ts b/src/test/console/consoleFormatTest.ts
index f7a9fd5e3..24a709e96 100644
--- a/src/test/console/consoleFormatTest.ts
+++ b/src/test/console/consoleFormatTest.ts
@@ -444,6 +444,17 @@ describe('console format', () => {
     p.assertLog();
   });
 
+  itIntegrates('json format', async ({ r }) => {
+    const p = await r.launchAndLoad('blank');
+    await p.logger.evaluateAndLog([
+      `console.log("%j", {host: "localhost"})`,
+      `console.log("%s: %s %j", "id", "Request headers", {host: "localhost"})`,
+      `console.log("%j", [1, 2, 3])`,
+      `console.log("%j", null)`,
+    ]);
+    p.assertLog();
+  });
+
   itIntegrates('colors', async ({ r }) => {
     const p = await r.launchAndLoad(`blank`);