diff --git a/src/js/_enqueues/lib/link.js b/src/js/_enqueues/lib/link.js
index a2500a9069648..6e4b82b3c0929 100644
--- a/src/js/_enqueues/lib/link.js
+++ b/src/js/_enqueues/lib/link.js
@@ -91,11 +91,16 @@
 			inputs.url.on( 'blur', wpLink.correctURL );
 		},
 
-		// If URL wasn't corrected last time and doesn't start with http:, https:, ? # or /, prepend http://.
+		/*
+		 * If the URL wasn't corrected last time and doesn't start with a scheme
+		 * (a letter followed by letters, digits, `+` or `-`, then a colon, matching
+		 * RFC 3986 minus the dot so that `example.com:8080` is still corrected),
+		 * `#`, `?`, `.` or `/`, prepend http://.
+		 */
 		correctURL: function () {
 			var url = inputs.url.val().trim();
 
-			if ( url && correctedURL !== url && ! /^(?:[a-z]+:|#|\?|\.|\/)/.test( url ) ) {
+			if ( url && correctedURL !== url && ! /^(?:[a-zA-Z][a-zA-Z0-9+-]*:|#|\?|\.|\/)/.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..8168f011cf802 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][a-zA-Z0-9+-]*:|#|\?|\.|\/)/.test( href ) && ! emailRegex.test( href ) ) {
 					href = 'http://' + href;
 				}
 
diff --git a/tests/qunit/index.html b/tests/qunit/index.html
index a6b6177014586..f9905bc924579 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>
@@ -167,6 +170,7 @@
 		<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/media/test-models.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..42492be9a4c3a
--- /dev/null
+++ b/tests/qunit/wp-includes/js/wplink.js
@@ -0,0 +1,98 @@
+/* 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 scheme containing a hyphen', function( assert ) {
+	jQuery( '#wp-link-url' ).val( 'chrome-extension://abcdef/options.html' );
+	wpLink.correctURL();
+	assert.strictEqual(
+		jQuery( '#wp-link-url' ).val(),
+		'chrome-extension://abcdef/options.html',
+		'A scheme containing a hyphen should not be modified.'
+	);
+} );
+
+QUnit.test( 'should not prepend http:// to a scheme containing a plus sign and digits', function( assert ) {
+	jQuery( '#wp-link-url' ).val( 'web+demo2://example.com' );
+	wpLink.correctURL();
+	assert.strictEqual(
+		jQuery( '#wp-link-url' ).val(),
+		'web+demo2://example.com',
+		'A scheme containing a plus sign and digits should not be modified.'
+	);
+} );
+
+QUnit.test( 'should prepend http:// to a bare host and port', function( assert ) {
+	jQuery( '#wp-link-url' ).val( 'example.com:8080/path' );
+	wpLink.correctURL();
+	assert.strictEqual(
+		jQuery( '#wp-link-url' ).val(),
+		'http://example.com:8080/path',
+		'A bare host and port is not a scheme, so it 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.'
+	);
+} );
