diff --git a/packages/@aws-cdk-testing/cli-integ/tests/cli-integ-tests/list/cdk-cdk-ls-ci-no-synth-time-on-stdout.integtest.ts b/packages/@aws-cdk-testing/cli-integ/tests/cli-integ-tests/list/cdk-cdk-ls-ci-no-synth-time-on-stdout.integtest.ts
new file mode 100644
index 000000000..c346830e1
--- /dev/null
+++ b/packages/@aws-cdk-testing/cli-integ/tests/cli-integ-tests/list/cdk-cdk-ls-ci-no-synth-time-on-stdout.integtest.ts
@@ -0,0 +1,22 @@
+import { integTest, withDefaultFixture } from '../../../lib';
+
+integTest(
+  'cdk ls --json in CI does not print synthesis time to stdout',
+  withDefaultFixture(async (fixture) => {
+    // `cdk ls --json` stdout is a machine-readable contract and is often piped (e.g. to `jq`),
+    // so it must be only the stack listing, not status lines like "✨ Synthesis time: ...".
+    const listing = await fixture.cdk(['ls', '--json'], {
+      verbose: false, // fixture defaults verbose on; turn it off so stdout is just the listing
+      captureStderr: false, // capture stdout only; stderr is folded into the result by default
+      modEnv: { CI: 'true' }, // CI routes non-error output to stdout (default is stderr)
+    });
+
+    const lines = listing.trim().split('\n').filter(line => line.length > 0);
+
+    // every line should be a stack; a synth-time line would not carry the prefix
+    expect(lines.length).toBeGreaterThan(0);
+    for (const line of lines) {
+      expect(line).toContain(fixture.stackNamePrefix);
+    }
+  }),
+);
diff --git a/packages/aws-cdk/lib/cli/cdk-toolkit.ts b/packages/aws-cdk/lib/cli/cdk-toolkit.ts
index 33209a5c9..e00ceb427 100644
--- a/packages/aws-cdk/lib/cli/cdk-toolkit.ts
+++ b/packages/aws-cdk/lib/cli/cdk-toolkit.ts
@@ -960,6 +960,11 @@ export class CdkToolkit {
   ): Promise<number> {
     this.ioHost.rewriteOnce(IO.CDK_TOOLKIT_I2901, (msg) => formatStackList(msg.data.stacks, options));
 
+    // With `--json`, stdout must stay machine-parsable, so suppress the synth-time line (I1000).
+    if (options.json) {
+      this.ioHost.once(IO.CDK_TOOLKIT_I1000, () => ({ preventDefault: true }));
+    }
+
     await this.toolkit.list(this.props.cloudExecutable, {
       stacks: selectors.length > 0
         ? { patterns: selectors, strategy: StackSelectionStrategy.PATTERN_MATCH, expand: ExpandStackSelection.UPSTREAM }
diff --git a/packages/aws-cdk/test/cli/cdk-toolkit.test.ts b/packages/aws-cdk/test/cli/cdk-toolkit.test.ts
index ddd61b59d..8543ad486 100644
--- a/packages/aws-cdk/test/cli/cdk-toolkit.test.ts
+++ b/packages/aws-cdk/test/cli/cdk-toolkit.test.ts
@@ -207,6 +207,35 @@ describe('list', () => {
       'Test-Stack-B',
     ]);
   });
+
+  test('with --json, suppresses the synthesis-time line so stdout stays machine-parsable', async () => {
+    // `cdk ls --json` stdout is a machine-readable contract; the "Synthesis time" line
+    // (CDK_TOOLKIT_I1000) must not be written (in CI mode non-error output goes to stdout).
+    const toolkit = defaultToolkitSetup();
+    const onceSpy = jest.spyOn(ioHost, 'once');
+
+    // WHEN
+    await toolkit.list([], { json: true });
+
+    // THEN - a one-shot suppressor for I1000 was registered that prevents default handling.
+    const i1000Call = onceSpy.mock.calls.find(([code]) => (code as any)?.code === 'CDK_TOOLKIT_I1000');
+    expect(i1000Call).toBeDefined();
+    const listener = i1000Call![1] as (msg: any) => any;
+    expect(listener({ code: 'CDK_TOOLKIT_I1000' })).toEqual({ preventDefault: true });
+  });
+
+  test('without --json, does not suppress the synthesis-time line', async () => {
+    // Plain `cdk ls` is not a machine-readable contract, so the line is left alone.
+    const toolkit = defaultToolkitSetup();
+    const onceSpy = jest.spyOn(ioHost, 'once');
+
+    // WHEN
+    await toolkit.list([]);
+
+    // THEN
+    const i1000Call = onceSpy.mock.calls.find(([code]) => (code as any)?.code === 'CDK_TOOLKIT_I1000');
+    expect(i1000Call).toBeUndefined();
+  });
 });
 
 describe('deploy', () => {