diff --git a/packages/react-devtools-shared/src/utils.js b/packages/react-devtools-shared/src/utils.js
index c412a0f562bcb..19b6ece3a75cd 100644
--- a/packages/react-devtools-shared/src/utils.js
+++ b/packages/react-devtools-shared/src/utils.js
@@ -11,8 +11,6 @@ import LRU from 'lru-cache';
 import {
   isElement,
   typeOf,
-  AsyncMode,
-  ConcurrentMode,
   ContextConsumer,
   ContextProvider,
   ForwardRef,
@@ -424,9 +422,6 @@ export function getDisplayNameForReactElement(
 ): string | null {
   const elementType = typeOf(element);
   switch (elementType) {
-    case AsyncMode:
-    case ConcurrentMode:
-      return 'ConcurrentMode';
     case ContextConsumer:
       return 'ContextConsumer';
     case ContextProvider:
diff --git a/packages/react-dom/src/server/ReactPartialRenderer.js b/packages/react-dom/src/server/ReactPartialRenderer.js
index 139c2f21e6dea..919d58a589967 100644
--- a/packages/react-dom/src/server/ReactPartialRenderer.js
+++ b/packages/react-dom/src/server/ReactPartialRenderer.js
@@ -30,7 +30,6 @@ import {
   REACT_FORWARD_REF_TYPE,
   REACT_FRAGMENT_TYPE,
   REACT_STRICT_MODE_TYPE,
-  REACT_CONCURRENT_MODE_TYPE,
   REACT_SUSPENSE_TYPE,
   REACT_SUSPENSE_LIST_TYPE,
   REACT_PORTAL_TYPE,
@@ -993,7 +992,6 @@ class ReactDOMServerRenderer {
 
       switch (elementType) {
         case REACT_STRICT_MODE_TYPE:
-        case REACT_CONCURRENT_MODE_TYPE:
         case REACT_PROFILER_TYPE:
         case REACT_SUSPENSE_LIST_TYPE:
         case REACT_FRAGMENT_TYPE: {
diff --git a/packages/react-is/src/ReactIs.js b/packages/react-is/src/ReactIs.js
index 0ca1ddc91b9f2..2b0b03e38d370 100644
--- a/packages/react-is/src/ReactIs.js
+++ b/packages/react-is/src/ReactIs.js
@@ -10,8 +10,6 @@
 'use strict';
 
 import {
-  REACT_ASYNC_MODE_TYPE,
-  REACT_CONCURRENT_MODE_TYPE,
   REACT_CONTEXT_TYPE,
   REACT_ELEMENT_TYPE,
   REACT_FORWARD_REF_TYPE,
@@ -34,8 +32,6 @@ export function typeOf(object: any) {
         const type = object.type;
 
         switch (type) {
-          case REACT_ASYNC_MODE_TYPE:
-          case REACT_CONCURRENT_MODE_TYPE:
           case REACT_FRAGMENT_TYPE:
           case REACT_PROFILER_TYPE:
           case REACT_STRICT_MODE_TYPE:
@@ -63,9 +59,6 @@ export function typeOf(object: any) {
   return undefined;
 }
 
-// AsyncMode is deprecated along with isAsyncMode
-export const AsyncMode = REACT_ASYNC_MODE_TYPE;
-export const ConcurrentMode = REACT_CONCURRENT_MODE_TYPE;
 export const ContextConsumer = REACT_CONTEXT_TYPE;
 export const ContextProvider = REACT_PROVIDER_TYPE;
 export const Element = REACT_ELEMENT_TYPE;
@@ -81,6 +74,7 @@ export const Suspense = REACT_SUSPENSE_TYPE;
 export {isValidElementType};
 
 let hasWarnedAboutDeprecatedIsAsyncMode = false;
+let hasWarnedAboutDeprecatedIsConcurrentMode = false;
 
 // AsyncMode should be deprecated
 export function isAsyncMode(object: any) {
@@ -90,15 +84,24 @@ export function isAsyncMode(object: any) {
       // Using console['warn'] to evade Babel and ESLint
       console['warn'](
         'The ReactIs.isAsyncMode() alias has been deprecated, ' +
-          'and will be removed in React 17+. Update your code to use ' +
-          'ReactIs.isConcurrentMode() instead. It has the exact same API.',
+          'and will be removed in React 17+.',
       );
     }
   }
-  return isConcurrentMode(object) || typeOf(object) === REACT_ASYNC_MODE_TYPE;
+  return false;
 }
 export function isConcurrentMode(object: any) {
-  return typeOf(object) === REACT_CONCURRENT_MODE_TYPE;
+  if (__DEV__) {
+    if (!hasWarnedAboutDeprecatedIsConcurrentMode) {
+      hasWarnedAboutDeprecatedIsConcurrentMode = true;
+      // Using console['warn'] to evade Babel and ESLint
+      console['warn'](
+        'The ReactIs.isConcurrentMode() alias has been deprecated, ' +
+          'and will be removed in React 17+.',
+      );
+    }
+  }
+  return false;
 }
 export function isContextConsumer(object: any) {
   return typeOf(object) === REACT_CONTEXT_TYPE;
diff --git a/packages/react-reconciler/src/ReactFiber.js b/packages/react-reconciler/src/ReactFiber.js
index ea41e2b10e829..3c181dfbd0cc5 100644
--- a/packages/react-reconciler/src/ReactFiber.js
+++ b/packages/react-reconciler/src/ReactFiber.js
@@ -84,7 +84,6 @@ import {
   REACT_PROFILER_TYPE,
   REACT_PROVIDER_TYPE,
   REACT_CONTEXT_TYPE,
-  REACT_CONCURRENT_MODE_TYPE,
   REACT_SUSPENSE_TYPE,
   REACT_SUSPENSE_LIST_TYPE,
   REACT_MEMO_TYPE,
@@ -638,10 +637,6 @@ export function createFiberFromTypeAndProps(
           expirationTime,
           key,
         );
-      case REACT_CONCURRENT_MODE_TYPE:
-        fiberTag = Mode;
-        mode |= ConcurrentMode | BlockingMode | StrictMode;
-        break;
       case REACT_STRICT_MODE_TYPE:
         fiberTag = Mode;
         mode |= StrictMode;
diff --git a/packages/shared/ReactSymbols.js b/packages/shared/ReactSymbols.js
index e05d8bf0f3bb7..527dd2ab42ce9 100644
--- a/packages/shared/ReactSymbols.js
+++ b/packages/shared/ReactSymbols.js
@@ -32,14 +32,6 @@ export const REACT_PROVIDER_TYPE = hasSymbol
 export const REACT_CONTEXT_TYPE = hasSymbol
   ? Symbol.for('react.context')
   : 0xeace;
-// TODO: We don't use AsyncMode or ConcurrentMode anymore. They were temporary
-// (unstable) APIs that have been removed. Can we remove the symbols?
-export const REACT_ASYNC_MODE_TYPE = hasSymbol
-  ? Symbol.for('react.async_mode')
-  : 0xeacf;
-export const REACT_CONCURRENT_MODE_TYPE = hasSymbol
-  ? Symbol.for('react.concurrent_mode')
-  : 0xeacf;
 export const REACT_FORWARD_REF_TYPE = hasSymbol
   ? Symbol.for('react.forward_ref')
   : 0xead0;
diff --git a/packages/shared/isValidElementType.js b/packages/shared/isValidElementType.js
index fb71c26971283..93a21b6570835 100644
--- a/packages/shared/isValidElementType.js
+++ b/packages/shared/isValidElementType.js
@@ -8,7 +8,6 @@
  */
 
 import {
-  REACT_CONCURRENT_MODE_TYPE,
   REACT_CONTEXT_TYPE,
   REACT_FORWARD_REF_TYPE,
   REACT_FRAGMENT_TYPE,
@@ -32,7 +31,6 @@ export default function isValidElementType(type: mixed) {
     typeof type === 'function' ||
     // Note: its typeof might be other than 'symbol' or 'number' if it's a polyfill.
     type === REACT_FRAGMENT_TYPE ||
-    type === REACT_CONCURRENT_MODE_TYPE ||
     type === REACT_PROFILER_TYPE ||
     type === REACT_STRICT_MODE_TYPE ||
     type === REACT_SUSPENSE_TYPE ||