diff --git a/fixtures/attribute-behavior/README.md b/fixtures/attribute-behavior/README.md
index fbf2acfb5230e..2411462f11aaa 100644
--- a/fixtures/attribute-behavior/README.md
+++ b/fixtures/attribute-behavior/README.md
@@ -28,4 +28,4 @@ Right now, we use a purple outline to call out cases where the assigned property
 
 This project was bootstrapped with [Create React App](https://github.com/facebook/create-react-app).
 
-You can find the guide for how to do things in a CRA [here](https://github.com/facebook/create-react-app/blob/master/packages/cra-template/template/README.md).
+You can find the guide for how to do things in a CRA [here](https://github.com/facebook/create-react-app/blob/main/packages/cra-template/template/README.md).
diff --git a/fixtures/concurrent/time-slicing/README.md b/fixtures/concurrent/time-slicing/README.md
index a61670e8512d8..2c2b90faa6cbd 100644
--- a/fixtures/concurrent/time-slicing/README.md
+++ b/fixtures/concurrent/time-slicing/README.md
@@ -10,7 +10,7 @@ It depends on a local build of React and enables us to easily test async "time s
 
 No. The APIs being tested here are unstable and some of them have still not been released to NPM. For now, this fixture is only a test harness.
 
-There are also known bugs and inefficiencies in master so **don't use this fixture for demonstration purposes either yet**. Until they are fixed, this fixture is **not** indicative of React async rendering performance.
+There are also known bugs and inefficiencies in main so **don't use this fixture for demonstration purposes either yet**. Until they are fixed, this fixture is **not** indicative of React async rendering performance.
 
 ## How do I run this fixture?
 
diff --git a/fixtures/devtools/regression/shared.js b/fixtures/devtools/regression/shared.js
index 055ba3d65e553..0010fd7ebbb58 100644
--- a/fixtures/devtools/regression/shared.js
+++ b/fixtures/devtools/regression/shared.js
@@ -32,7 +32,7 @@ if (typeof SchedulerTracing !== 'undefined') {
   trace = (_, __, callback) => callback();
 }
 
-// https://github.com/facebook/react/blob/master/CHANGELOG.md
+// https://github.com/facebook/react/blob/main/CHANGELOG.md
 switch (major) {
   case 16:
     switch (minor) {
diff --git a/packages/react-devtools-extensions/flow-typed/npm/react-test-renderer_v16.x.x.js b/packages/react-devtools-extensions/flow-typed/npm/react-test-renderer_v16.x.x.js
index 585b215bf68ef..cd2ae506d2ef5 100644
--- a/packages/react-devtools-extensions/flow-typed/npm/react-test-renderer_v16.x.x.js
+++ b/packages/react-devtools-extensions/flow-typed/npm/react-test-renderer_v16.x.x.js
@@ -2,7 +2,7 @@
 // flow-typed version: 9c71eca8ef/react-test-renderer_v16.x.x/flow_>=v0.47.x
 
 // Type definitions for react-test-renderer 16.x.x
-// Ported from: https://github.com/DefinitelyTyped/DefinitelyTyped/blob/main/types/react-test-renderer
+// Ported from: https://github.com/DefinitelyTyped/DefinitelyTyped/blob/master/types/react-test-renderer
 
 'use strict';
 
diff --git a/packages/react-devtools-shared/src/__tests__/profilingCache-test.js b/packages/react-devtools-shared/src/__tests__/profilingCache-test.js
index 2cf31e867b57f..ace43294e9315 100644
--- a/packages/react-devtools-shared/src/__tests__/profilingCache-test.js
+++ b/packages/react-devtools-shared/src/__tests__/profilingCache-test.js
@@ -672,7 +672,7 @@ describe('ProfilingCache', () => {
 
     const About = () => <div>About</div>;
 
-    // Mimics https://github.com/ReactTraining/react-router/blob/main/packages/react-router/modules/Router.js
+    // Mimics https://github.com/ReactTraining/react-router/blob/master/packages/react-router/modules/Router.js
     function Router({children}) {
       const [path, setPath] = React.useState('/');
       return (
@@ -682,7 +682,7 @@ describe('ProfilingCache', () => {
       );
     }
 
-    // Mimics https://github.com/ReactTraining/react-router/blob/main/packages/react-router/modules/Switch.js
+    // Mimics https://github.com/ReactTraining/react-router/blob/master/packages/react-router/modules/Switch.js
     function Switch({children}) {
       return (
         <RouterContext.Consumer>
@@ -699,14 +699,14 @@ describe('ProfilingCache', () => {
       );
     }
 
-    // Mimics https://github.com/ReactTraining/react-router/blob/main/packages/react-router/modules/Route.js
+    // Mimics https://github.com/ReactTraining/react-router/blob/master/packages/react-router/modules/Route.js
     function Route({children, path}) {
       return null;
     }
 
     const linkRef = React.createRef();
 
-    // Mimics https://github.com/ReactTraining/react-router/blob/main/packages/react-router-dom/modules/Link.js
+    // Mimics https://github.com/ReactTraining/react-router/blob/master/packages/react-router-dom/modules/Link.js
     function Link({children, path}) {
       return (
         <RouterContext.Consumer>
diff --git a/packages/react-devtools/CHANGELOG.md b/packages/react-devtools/CHANGELOG.md
index c3c92ea8e0615..86aa2e2e61a8e 100644
--- a/packages/react-devtools/CHANGELOG.md
+++ b/packages/react-devtools/CHANGELOG.md
@@ -66,7 +66,7 @@
 * Console patching should handle Symbols without erroring ([bvaughn](https://github.com/bvaughn) in [#21368](https://github.com/facebook/react/pull/21368))
 
 ###### Bridge protocol version backend/frontend
-During initialization, DevTools now checks to ensure it's compatible with the ["backend"](https://github.com/facebook/react/blob/master/packages/react-devtools/OVERVIEW.md#overview) that's embedded within a renderer like React Native. If the two aren't compatible, upgrade instructions will be shown:
+During initialization, DevTools now checks to ensure it's compatible with the ["backend"](https://github.com/facebook/react/blob/main/packages/react-devtools/OVERVIEW.md#overview) that's embedded within a renderer like React Native. If the two aren't compatible, upgrade instructions will be shown:
 
 <img width="400" height="233" alt="Dialog displaying downgrade instructions for the React DevTools frontend to connect to an older backend version" src="https://user-images.githubusercontent.com/29597/115997927-f77f2a00-a5b2-11eb-9098-20042b664cea.png">
     
diff --git a/scripts/error-codes/extract-errors.js b/scripts/error-codes/extract-errors.js
index 4541a3eeeab6c..58823cbe16873 100644
--- a/scripts/error-codes/extract-errors.js
+++ b/scripts/error-codes/extract-errors.js
@@ -18,7 +18,7 @@ const babylonOptions = {
   // As a parser, babylon has its own options and we can't directly
   // import/require a babel preset. It should be kept **the same** as
   // the `babel-plugin-syntax-*` ones specified in
-  // https://github.com/facebook/fbjs/blob/main/packages/babel-preset-fbjs/configure.js
+  // https://github.com/facebook/fbjs/blob/master/packages/babel-preset-fbjs/configure.js
   plugins: [
     'classProperties',
     'flow',