diff --git a/src/js/_enqueues/lib/link.js b/src/js/_enqueues/lib/link.js
index a2500a9069648..a26785929647c 100644
--- a/src/js/_enqueues/lib/link.js
+++ b/src/js/_enqueues/lib/link.js
@@ -95,7 +95,7 @@
 		correctURL: function () {
 			var url = inputs.url.val().trim();
 
-			if ( url && correctedURL !== url && ! /^(?:[a-z]+:|#|\?|\.|\/)/.test( url ) ) {
+			if ( url && correctedURL !== url && ! /^(?:[a-zA-Z]+:|#|\?|\.|\/)/.test( url ) ) {
 				inputs.url.val( 'http://' + url );
 				correctedURL = url;
 			}
diff --git a/src/js/_enqueues/vendor/tinymce/plugins/wplink/plugin.js b/src/js/_enqueues/vendor/tinymce/plugins/wplink/plugin.js
index 76781894171e8..3f498fb3ee3a0 100644
--- a/src/js/_enqueues/vendor/tinymce/plugins/wplink/plugin.js
+++ b/src/js/_enqueues/vendor/tinymce/plugins/wplink/plugin.js
@@ -264,7 +264,7 @@
 					return;
 				}
 
-				if ( ! /^(?:[a-z]+:|#|\?|\.|\/)/.test( href ) && ! emailRegex.test( href ) ) {
+				if ( ! /^(?:[a-zA-Z]+:|#|\?|\.|\/)/.test( href ) && ! emailRegex.test( href ) ) {
 					href = 'http://' + href;
 				}
 
diff --git a/tests/qunit/index.html b/tests/qunit/index.html
index 9fd35f0c1ffc2..0fe52dd83d6a6 100644
--- a/tests/qunit/index.html
+++ b/tests/qunit/index.html
@@ -76,6 +76,7 @@
 	<body>
 		<div id="qunit"></div>
 		<div id="qunit-fixture">
+			<input id="wp-link-url" type="url" />
 			<script src="fixtures/customize-header.js"></script>
 			<script src="fixtures/customize-settings.js"></script>
 			<script src="fixtures/customize-menus.js"></script>
@@ -87,6 +88,8 @@
 		</div>
 
 		<!-- Tested files -->
+		<script>window.wpLinkL10n = window.wpLinkL10n || {};</script>
+		<script src="../../build/wp-includes/js/wplink.js"></script>
 		<script src="../../build/wp-admin/js/dashboard.js"></script>
 		<script src="../../build/wp-admin/js/password-strength-meter.js"></script>
 		<script src="../../build/wp-admin/js/postbox.js"></script>
@@ -166,6 +169,7 @@
 		<script src="wp-admin/js/widgets/test-media-image-widget.js"></script>
 		<script src="wp-admin/js/widgets/test-media-gallery-widget.js"></script>
 		<script src="wp-admin/js/widgets/test-media-video-widget.js"></script>
+		<script src="wp-includes/js/wplink.js"></script>
 
 		<!-- Customizer templates for sections -->
 		<script type="text/html" id="tmpl-customize-section-default">
diff --git a/tests/qunit/wp-includes/js/wplink.js b/tests/qunit/wp-includes/js/wplink.js
new file mode 100644
index 0000000000000..e53c43d5cf064
--- /dev/null
+++ b/tests/qunit/wp-includes/js/wplink.js
@@ -0,0 +1,68 @@
+/* global wpLink, jQuery */
+
+QUnit.module( 'wpLink.correctURL', {
+	beforeEach: function() {
+		// Re-initialize after QUnit resets the fixture, so inputs.url references the live element.
+		wpLink.init();
+	}
+} );
+
+QUnit.test( 'should not prepend http:// to a lowercase https:// URL', function( assert ) {
+	jQuery( '#wp-link-url' ).val( 'https://example.com' );
+	wpLink.correctURL();
+	assert.strictEqual(
+		jQuery( '#wp-link-url' ).val(),
+		'https://example.com',
+		'A lowercase https:// URL should not be modified.'
+	);
+} );
+
+QUnit.test( 'should not prepend http:// to an uppercase HTTPS:// URL', function( assert ) {
+	jQuery( '#wp-link-url' ).val( 'HTTPS://example.net' );
+	wpLink.correctURL();
+	assert.strictEqual(
+		jQuery( '#wp-link-url' ).val(),
+		'HTTPS://example.net',
+		'An uppercase HTTPS:// URL should not be modified (common on mobile keyboards).'
+	);
+} );
+
+QUnit.test( 'should not prepend http:// to a mixed-case Http:// URL', function( assert ) {
+	jQuery( '#wp-link-url' ).val( 'Http://example.org' );
+	wpLink.correctURL();
+	assert.strictEqual(
+		jQuery( '#wp-link-url' ).val(),
+		'Http://example.org',
+		'A mixed-case Http:// URL should not be modified.'
+	);
+} );
+
+QUnit.test( 'should prepend http:// to a bare domain without a scheme', function( assert ) {
+	jQuery( '#wp-link-url' ).val( 'example.com' );
+	wpLink.correctURL();
+	assert.strictEqual(
+		jQuery( '#wp-link-url' ).val(),
+		'http://example.com',
+		'A bare domain should get http:// prepended.'
+	);
+} );
+
+QUnit.test( 'should not prepend http:// to a fragment URL', function( assert ) {
+	jQuery( '#wp-link-url' ).val( '#section' );
+	wpLink.correctURL();
+	assert.strictEqual(
+		jQuery( '#wp-link-url' ).val(),
+		'#section',
+		'A fragment URL should not be modified.'
+	);
+} );
+
+QUnit.test( 'should not prepend http:// to a root-relative URL', function( assert ) {
+	jQuery( '#wp-link-url' ).val( '/page/subpage/' );
+	wpLink.correctURL();
+	assert.strictEqual(
+		jQuery( '#wp-link-url' ).val(),
+		'/page/subpage/',
+		'A root-relative URL should not be modified.'
+	);
+} );
