diff --git a/packages/react-dom/src/__tests__/ReactServerRendering-test.js b/packages/react-dom/src/__tests__/ReactServerRendering-test.js
index 86519ae5854a9..2e3bd87d6a59c 100644
--- a/packages/react-dom/src/__tests__/ReactServerRendering-test.js
+++ b/packages/react-dom/src/__tests__/ReactServerRendering-test.js
@@ -760,12 +760,12 @@ describe('ReactDOMServer', () => {
     );
     expect(console.error.calls.count()).toBe(2);
     expect(console.error.calls.argsFor(0)[0]).toBe(
-      'Warning: <inPUT /> is using uppercase HTML. Always use lowercase ' +
+      'Warning: <inPUT /> is using lowercase HTML. Always use uppercase ' +
         'HTML tags in React.',
     );
     // linearGradient doesn't warn
     expect(console.error.calls.argsFor(1)[0]).toBe(
-      'Warning: <iFrame /> is using uppercase HTML. Always use lowercase ' +
+      'Warning: <iFrame /> is using lowercase HTML. Always use uppercase ' +
         'HTML tags in React.',
     );
   });
diff --git a/packages/react-dom/src/client/ReactDOMFiberComponent.js b/packages/react-dom/src/client/ReactDOMFiberComponent.js
index e6ff7e7c92cb2..f8c6e31037dd3 100644
--- a/packages/react-dom/src/client/ReactDOMFiberComponent.js
+++ b/packages/react-dom/src/client/ReactDOMFiberComponent.js
@@ -373,7 +373,7 @@ export function createElement(
       // allow <SVG> or <mATH>.
       warning(
         isCustomComponentTag || type === type.toLowerCase(),
-        '<%s /> is using uppercase HTML. Always use lowercase HTML tags ' +
+        '<%s /> is using lowercase HTML. Always use uppercase HTML tags ' +
           'in React.',
         type,
       );
diff --git a/packages/react-dom/src/server/ReactPartialRenderer.js b/packages/react-dom/src/server/ReactPartialRenderer.js
index 5568ab504ef28..8d1c85a4cc218 100644
--- a/packages/react-dom/src/server/ReactPartialRenderer.js
+++ b/packages/react-dom/src/server/ReactPartialRenderer.js
@@ -644,7 +644,7 @@ class ReactDOMServerRenderer {
         // allow <SVG> or <mATH>.
         warning(
           tag === element.type,
-          '<%s /> is using uppercase HTML. Always use lowercase HTML tags ' +
+          '<%s /> is using lowercase HTML. Always use uppercase HTML tags ' +
             'in React.',
           element.type,
         );