diff --git a/core/theme.js b/core/theme.js
index d156b1227d9..e5bc08c52bd 100644
--- a/core/theme.js
+++ b/core/theme.js
@@ -149,6 +149,7 @@ class Theme {
    * @return {!Theme} A new Blockly theme.
    */
   static defineTheme(name, themeObj) {
+    name = name.toLowerCase();
     const theme = new Theme(name);
     let base = themeObj['base'];
     if (base) {
diff --git a/tests/mocha/theme_test.js b/tests/mocha/theme_test.js
index 95d61ce0d04..cc29797f112 100644
--- a/tests/mocha/theme_test.js
+++ b/tests/mocha/theme_test.js
@@ -274,4 +274,11 @@ suite('Theme', function() {
           this.constants.validatedBlockStyle_(inputStyle), expectedOutput);
     });
   });
+
+  suite('defineTheme', function() {
+    test('Normalizes to lowercase', function() {
+      const theme = Blockly.Theme.defineTheme('TEST', {});
+      chai.assert.equal(theme.name, 'test');
+    });
+  });
 });