diff --git a/packages/react-devtools-scheduling-profiler/src/import-worker/__tests__/preprocessData-test.internal.js b/packages/react-devtools-scheduling-profiler/src/import-worker/__tests__/preprocessData-test.internal.js
index e24611e9daea5..e458f8060f84b 100644
--- a/packages/react-devtools-scheduling-profiler/src/import-worker/__tests__/preprocessData-test.internal.js
+++ b/packages/react-devtools-scheduling-profiler/src/import-worker/__tests__/preprocessData-test.internal.js
@@ -17,6 +17,8 @@ import {
 } from '../../constants';
 import REACT_VERSION from 'shared/ReactVersion';
 
+global.IS_REACT_ACT_ENVIRONMENT = true;
+
 describe('getLanesFromTransportDecimalBitmask', () => {
   it('should return array of lane numbers from bitmask string', () => {
     expect(getLanesFromTransportDecimalBitmask('1')).toEqual([0]);
diff --git a/packages/react-devtools-shared/src/__tests__/inspectedElement-test.js b/packages/react-devtools-shared/src/__tests__/inspectedElement-test.js
index 68d014de3ad47..be5a852396538 100644
--- a/packages/react-devtools-shared/src/__tests__/inspectedElement-test.js
+++ b/packages/react-devtools-shared/src/__tests__/inspectedElement-test.js
@@ -38,6 +38,8 @@ describe('InspectedElement', () => {
   let ErrorBoundary;
   let errorBoundaryInstance;
 
+  global.IS_REACT_ACT_ENVIRONMENT = true;
+
   beforeEach(() => {
     utils = require('./utils');
     utils.beforeEachProfiling();
diff --git a/packages/react-dom/src/__tests__/ReactDOMNativeEventHeuristic-test.js b/packages/react-dom/src/__tests__/ReactDOMNativeEventHeuristic-test.js
index 702f213d7a2ed..94c5fe35290bd 100644
--- a/packages/react-dom/src/__tests__/ReactDOMNativeEventHeuristic-test.js
+++ b/packages/react-dom/src/__tests__/ReactDOMNativeEventHeuristic-test.js
@@ -76,9 +76,7 @@ describe('ReactDOMNativeEventHeuristic-test', () => {
     // Dispatch a click event on the Disable-button.
     const firstEvent = document.createEvent('Event');
     firstEvent.initEvent('click', true, true);
-    expect(() =>
-      dispatchAndSetCurrentEvent(disableButton, firstEvent),
-    ).toErrorDev(['An update to Form inside a test was not wrapped in act']);
+    dispatchAndSetCurrentEvent(disableButton, firstEvent);
 
     // Discrete events should be flushed in a microtask.
     // Verify that the second button was removed.
@@ -134,9 +132,7 @@ describe('ReactDOMNativeEventHeuristic-test', () => {
     // Dispatch a click event on the Disable-button.
     const firstEvent = document.createEvent('Event');
     firstEvent.initEvent('click', true, true);
-    expect(() => {
-      dispatchAndSetCurrentEvent(disableButton, firstEvent);
-    }).toErrorDev(['An update to Form inside a test was not wrapped in act']);
+    dispatchAndSetCurrentEvent(disableButton, firstEvent);
 
     // There should now be a pending update to disable the form.
     // This should not have flushed yet since it's in concurrent mode.
@@ -196,9 +192,7 @@ describe('ReactDOMNativeEventHeuristic-test', () => {
     // Dispatch a click event on the Enable-button.
     const firstEvent = document.createEvent('Event');
     firstEvent.initEvent('click', true, true);
-    expect(() => {
-      dispatchAndSetCurrentEvent(enableButton, firstEvent);
-    }).toErrorDev(['An update to Form inside a test was not wrapped in act']);
+    dispatchAndSetCurrentEvent(enableButton, firstEvent);
 
     // There should now be a pending update to enable the form.
     // This should not have flushed yet since it's in concurrent mode.
@@ -344,9 +338,6 @@ describe('ReactDOMNativeEventHeuristic-test', () => {
     });
     expect(container.textContent).toEqual('Count: 0');
 
-    // Ignore act warning. We can't use act because it forces batched updates.
-    spyOnDev(console, 'error');
-
     const pressEvent = document.createEvent('Event');
     pressEvent.initEvent('click', true, true);
     dispatchAndSetCurrentEvent(target.current, pressEvent);
@@ -355,17 +346,6 @@ describe('ReactDOMNativeEventHeuristic-test', () => {
     await null;
     // If this is 2, that means the `setCount` calls were not batched.
     expect(container.textContent).toEqual('Count: 1');
-
-    // Assert that the `act` warnings were the only ones that fired.
-    if (__DEV__) {
-      expect(console.error).toHaveBeenCalledTimes(2);
-      expect(console.error.calls.argsFor(0)[0]).toContain(
-        'was not wrapped in act',
-      );
-      expect(console.error.calls.argsFor(1)[0]).toContain(
-        'was not wrapped in act',
-      );
-    }
   });
 
   it('should not flush discrete events at the end of outermost batchedUpdates', async () => {
diff --git a/packages/react-dom/src/__tests__/ReactTestUtilsAct-test.js b/packages/react-dom/src/__tests__/ReactTestUtilsAct-test.js
index f5e33d789891c..d0992dce53d41 100644
--- a/packages/react-dom/src/__tests__/ReactTestUtilsAct-test.js
+++ b/packages/react-dom/src/__tests__/ReactTestUtilsAct-test.js
@@ -16,6 +16,8 @@ let container;
 
 jest.useRealTimers();
 
+global.IS_REACT_ACT_ENVIRONMENT = true;
+
 function sleep(period) {
   return new Promise(resolve => {
     setTimeout(() => {
diff --git a/packages/react-reconciler/src/ReactFiberAct.new.js b/packages/react-reconciler/src/ReactFiberAct.new.js
index 4338a7d5cd459..18055e7c738c7 100644
--- a/packages/react-reconciler/src/ReactFiberAct.new.js
+++ b/packages/react-reconciler/src/ReactFiberAct.new.js
@@ -8,7 +8,13 @@
  */
 
 import type {Fiber} from './ReactFiber.new';
+
+import ReactSharedInternals from 'shared/ReactSharedInternals';
+
 import {warnsIfNotActing} from './ReactFiberHostConfig';
+import {ConcurrentMode} from './ReactTypeOfMode';
+
+const {ReactCurrentActQueue} = ReactSharedInternals;
 
 export function isActEnvironment(fiber: Fiber) {
   if (__DEV__) {
@@ -18,18 +24,31 @@ export function isActEnvironment(fiber: Fiber) {
         ? IS_REACT_ACT_ENVIRONMENT
         : undefined;
 
-    // TODO: Only check `jest` in legacy mode. In concurrent mode, this
-    // heuristic is replaced by IS_REACT_ACT_ENVIRONMENT.
-    // $FlowExpectedError - Flow doesn't know about jest
-    const jestIsDefined = typeof jest !== 'undefined';
-    return (
-      warnsIfNotActing &&
-      jestIsDefined &&
-      // Legacy mode assumes an act environment whenever `jest` is defined, but
-      // you can still turn off spurious warnings by setting
-      // IS_REACT_ACT_ENVIRONMENT explicitly to false.
-      isReactActEnvironmentGlobal !== false
-    );
+    if (fiber.mode & ConcurrentMode) {
+      if (
+        !isReactActEnvironmentGlobal &&
+        ReactCurrentActQueue.current !== null
+      ) {
+        // TODO: Include link to relevant documentation page.
+        console.error(
+          'The current testing environment is not configured to support ' +
+            'act(...)',
+        );
+      }
+      return isReactActEnvironmentGlobal;
+    } else {
+      // Legacy mode. We preserve the behavior of React 17's act. It assumes an
+      // act environment whenever `jest` is defined, but you can still turn off
+      // spurious warnings by setting IS_REACT_ACT_ENVIRONMENT explicitly
+      // to false.
+      // $FlowExpectedError - Flow doesn't know about jest
+      const jestIsDefined = typeof jest !== 'undefined';
+      return (
+        warnsIfNotActing &&
+        jestIsDefined &&
+        isReactActEnvironmentGlobal !== false
+      );
+    }
   }
   return false;
 }
diff --git a/packages/react-reconciler/src/ReactFiberAct.old.js b/packages/react-reconciler/src/ReactFiberAct.old.js
index e477024a858f8..ddae518dcb631 100644
--- a/packages/react-reconciler/src/ReactFiberAct.old.js
+++ b/packages/react-reconciler/src/ReactFiberAct.old.js
@@ -8,7 +8,13 @@
  */
 
 import type {Fiber} from './ReactFiber.old';
+
+import ReactSharedInternals from 'shared/ReactSharedInternals';
+
 import {warnsIfNotActing} from './ReactFiberHostConfig';
+import {ConcurrentMode} from './ReactTypeOfMode';
+
+const {ReactCurrentActQueue} = ReactSharedInternals;
 
 export function isActEnvironment(fiber: Fiber) {
   if (__DEV__) {
@@ -18,18 +24,31 @@ export function isActEnvironment(fiber: Fiber) {
         ? IS_REACT_ACT_ENVIRONMENT
         : undefined;
 
-    // TODO: Only check `jest` in legacy mode. In concurrent mode, this
-    // heuristic is replaced by IS_REACT_ACT_ENVIRONMENT.
-    // $FlowExpectedError - Flow doesn't know about jest
-    const jestIsDefined = typeof jest !== 'undefined';
-    return (
-      warnsIfNotActing &&
-      jestIsDefined &&
-      // Legacy mode assumes an act environment whenever `jest` is defined, but
-      // you can still turn off spurious warnings by setting
-      // IS_REACT_ACT_ENVIRONMENT explicitly to false.
-      isReactActEnvironmentGlobal !== false
-    );
+    if (fiber.mode & ConcurrentMode) {
+      if (
+        !isReactActEnvironmentGlobal &&
+        ReactCurrentActQueue.current !== null
+      ) {
+        // TODO: Include link to relevant documentation page.
+        console.error(
+          'The current testing environment is not configured to support ' +
+            'act(...)',
+        );
+      }
+      return isReactActEnvironmentGlobal;
+    } else {
+      // Legacy mode. We preserve the behavior of React 17's act. It assumes an
+      // act environment whenever `jest` is defined, but you can still turn off
+      // spurious warnings by setting IS_REACT_ACT_ENVIRONMENT explicitly
+      // to false.
+      // $FlowExpectedError - Flow doesn't know about jest
+      const jestIsDefined = typeof jest !== 'undefined';
+      return (
+        warnsIfNotActing &&
+        jestIsDefined &&
+        isReactActEnvironmentGlobal !== false
+      );
+    }
   }
   return false;
 }
diff --git a/packages/react-reconciler/src/__tests__/DebugTracing-test.internal.js b/packages/react-reconciler/src/__tests__/DebugTracing-test.internal.js
index 94431b435054e..ee93b3f992994 100644
--- a/packages/react-reconciler/src/__tests__/DebugTracing-test.internal.js
+++ b/packages/react-reconciler/src/__tests__/DebugTracing-test.internal.js
@@ -19,6 +19,8 @@ describe('DebugTracing', () => {
   const DEFAULT_LANE_STRING = '0b0000000000000000000000000010000';
   const RETRY_LANE_STRING = '0b0000000010000000000000000000000';
 
+  global.IS_REACT_ACT_ENVIRONMENT = true;
+
   beforeEach(() => {
     jest.resetModules();
 
diff --git a/packages/react-reconciler/src/__tests__/ReactActWarnings-test.js b/packages/react-reconciler/src/__tests__/ReactActWarnings-test.js
new file mode 100644
index 0000000000000..058e01b1fe060
--- /dev/null
+++ b/packages/react-reconciler/src/__tests__/ReactActWarnings-test.js
@@ -0,0 +1,130 @@
+/**
+ * Copyright (c) Facebook, Inc. and its affiliates.
+ *
+ * This source code is licensed under the MIT license found in the
+ * LICENSE file in the root directory of this source tree.
+ *
+ * @jest-environment node
+ */
+
+let React;
+let Scheduler;
+let ReactNoop;
+let useState;
+let act;
+
+// These tests are mostly concerned with concurrent roots. The legacy root
+// behavior is covered by other older test suites and is unchanged from
+// React 17.
+describe('act warnings', () => {
+  beforeEach(() => {
+    jest.resetModules();
+    React = require('react');
+    Scheduler = require('scheduler');
+    ReactNoop = require('react-noop-renderer');
+    act = React.unstable_act;
+    useState = React.useState;
+  });
+
+  function Text(props) {
+    Scheduler.unstable_yieldValue(props.text);
+    return props.text;
+  }
+
+  function withActEnvironment(value, scope) {
+    const prevValue = global.IS_REACT_ACT_ENVIRONMENT;
+    global.IS_REACT_ACT_ENVIRONMENT = value;
+    try {
+      return scope();
+    } finally {
+      global.IS_REACT_ACT_ENVIRONMENT = prevValue;
+    }
+  }
+
+  test('warns about unwrapped updates only if environment flag is enabled', () => {
+    let setState;
+    function App() {
+      const [state, _setState] = useState(0);
+      setState = _setState;
+      return <Text text={state} />;
+    }
+
+    const root = ReactNoop.createRoot();
+    root.render(<App />);
+    expect(Scheduler).toFlushAndYield([0]);
+    expect(root).toMatchRenderedOutput('0');
+
+    // Default behavior. Flag is undefined. No warning.
+    expect(global.IS_REACT_ACT_ENVIRONMENT).toBe(undefined);
+    setState(1);
+    expect(Scheduler).toFlushAndYield([1]);
+    expect(root).toMatchRenderedOutput('1');
+
+    // Flag is true. Warn.
+    withActEnvironment(true, () => {
+      expect(() => setState(2)).toErrorDev(
+        'An update to App inside a test was not wrapped in act',
+      );
+      expect(Scheduler).toFlushAndYield([2]);
+      expect(root).toMatchRenderedOutput('2');
+    });
+
+    // Flag is false. No warning.
+    withActEnvironment(false, () => {
+      setState(3);
+      expect(Scheduler).toFlushAndYield([3]);
+      expect(root).toMatchRenderedOutput('3');
+    });
+  });
+
+  // @gate __DEV__
+  test('act warns if the environment flag is not enabled', () => {
+    let setState;
+    function App() {
+      const [state, _setState] = useState(0);
+      setState = _setState;
+      return <Text text={state} />;
+    }
+
+    const root = ReactNoop.createRoot();
+    root.render(<App />);
+    expect(Scheduler).toFlushAndYield([0]);
+    expect(root).toMatchRenderedOutput('0');
+
+    // Default behavior. Flag is undefined. Warn.
+    expect(global.IS_REACT_ACT_ENVIRONMENT).toBe(undefined);
+    expect(() => {
+      act(() => {
+        setState(1);
+      });
+    }).toErrorDev(
+      'The current testing environment is not configured to support act(...)',
+      {withoutStack: true},
+    );
+    expect(Scheduler).toHaveYielded([1]);
+    expect(root).toMatchRenderedOutput('1');
+
+    // Flag is true. Don't warn.
+    withActEnvironment(true, () => {
+      act(() => {
+        setState(2);
+      });
+      expect(Scheduler).toHaveYielded([2]);
+      expect(root).toMatchRenderedOutput('2');
+    });
+
+    // Flag is false. Warn.
+    withActEnvironment(false, () => {
+      expect(() => {
+        act(() => {
+          setState(1);
+        });
+      }).toErrorDev(
+        'The current testing environment is not configured to support act(...)',
+        {withoutStack: true},
+      );
+      expect(Scheduler).toHaveYielded([1]);
+      expect(root).toMatchRenderedOutput('1');
+    });
+  });
+});
diff --git a/packages/react-reconciler/src/__tests__/SchedulingProfilerLabels-test.internal.js b/packages/react-reconciler/src/__tests__/SchedulingProfilerLabels-test.internal.js
index 09fdb95b01e6c..13ca3988b2a71 100644
--- a/packages/react-reconciler/src/__tests__/SchedulingProfilerLabels-test.internal.js
+++ b/packages/react-reconciler/src/__tests__/SchedulingProfilerLabels-test.internal.js
@@ -21,6 +21,8 @@ describe('SchedulingProfiler labels', () => {
   let featureDetectionMarkName = null;
   let marks;
 
+  global.IS_REACT_ACT_ENVIRONMENT = true;
+
   function polyfillJSDomUserTiming() {
     featureDetectionMarkName = null;