Index: src/wp-includes/class-wp-editor.php
===================================================================
--- src/wp-includes/class-wp-editor.php	(revision 38797)
+++ src/wp-includes/class-wp-editor.php	(working copy)
@@ -600,7 +600,7 @@
 				$mce_buttons = apply_filters( 'teeny_mce_buttons', array('bold', 'italic', 'underline', 'blockquote', 'strikethrough', 'bullist', 'numlist', 'alignleft', 'aligncenter', 'alignright', 'undo', 'redo', 'link', 'unlink', 'fullscreen'), $editor_id );
 				$mce_buttons_2 = $mce_buttons_3 = $mce_buttons_4 = array();
 			} else {
-				$mce_buttons = array( 'bold', 'italic', 'strikethrough', 'bullist', 'numlist', 'blockquote', 'hr', 'alignleft', 'aligncenter', 'alignright', 'link', 'unlink', 'wp_more', 'spellchecker' );
+				$mce_buttons = array( 'bold', 'italic', 'strikethrough', 'wp_mark', 'bullist', 'numlist', 'blockquote', 'hr', 'alignleft', 'aligncenter', 'alignright', 'link', 'unlink', 'wp_more', 'spellchecker' );
 
 				if ( ! wp_is_mobile() ) {
 					if ( $set['_content_editor_dfw'] ) {
@@ -622,7 +622,7 @@
 				 */
 				$mce_buttons = apply_filters( 'mce_buttons', $mce_buttons, $editor_id );
 
-				$mce_buttons_2 = array( 'formatselect', 'underline', 'alignjustify', 'forecolor', 'pastetext', 'removeformat', 'charmap', 'outdent', 'indent', 'undo', 'redo' );
+				$mce_buttons_2 = array( 'formatselect', 'alignjustify', 'forecolor', 'pastetext', 'removeformat', 'charmap', 'outdent', 'indent', 'undo', 'redo' );
 
 				if ( ! wp_is_mobile() ) {
 					$mce_buttons_2[] = 'wp_help';
@@ -1049,6 +1049,9 @@
 			'Apply'  => __( 'Apply' ), // Tooltip for the 'apply' button in the inline link dialog
 			'Link options'  => __( 'Link options' ), // Tooltip for the 'link options' button in the inline link dialog
 
+			/* translators: as in marking/highlighting text */
+			'Mark' => _x( 'Mark', 'TinyMCE button' ),
+
 			// Shortcuts help modal
 			'Keyboard Shortcuts' => __( 'Keyboard Shortcuts' ),
 			'Default shortcuts,' => __( 'Default shortcuts,' ),
Index: src/wp-includes/css/editor.css
===================================================================
--- src/wp-includes/css/editor.css	(revision 38797)
+++ src/wp-includes/css/editor.css	(working copy)
@@ -888,6 +888,7 @@
 i.mce-i-wp_page,
 i.mce-i-hr,
 i.mce-i-wp_code,
+i.mce-i-wp_mark,
 i.mce-i-dashicon,
 i.mce-i-remove {
 	font: normal 20px/1 dashicons;
@@ -1041,6 +1042,10 @@
 	content: "\f475";
 }
 
+i.mce-i-wp_mark:before {
+	content: "\f540";
+}
+
 /* RTL button icons */
 .rtl i.mce-i-outdent:before {
 	content: "\f222";
Index: src/wp-includes/js/tinymce/plugins/wordpress/plugin.js
===================================================================
--- src/wp-includes/js/tinymce/plugins/wordpress/plugin.js	(revision 38797)
+++ src/wp-includes/js/tinymce/plugins/wordpress/plugin.js	(working copy)
@@ -222,6 +222,10 @@
 		editor.formatter.toggle('code');
 	});
 
+	editor.addCommand( 'WP_Mark', function() {
+		editor.formatter.toggle('mark');
+	});
+
 	editor.addCommand( 'WP_Page', function() {
 		editor.execCommand( 'WP_More', 'nextpage' );
 	});
@@ -389,6 +393,12 @@
 		stateSelector: 'code'
 	});
 
+	editor.addButton( 'wp_mark', {
+		tooltip: 'Mark',
+		cmd: 'WP_Mark',
+		stateSelector: 'mark'
+	});
+
 	// Menubar
 	// Insert->Add Media
 	if ( wp && wp.media && wp.media.editor ) {
@@ -521,6 +531,10 @@
 				});
 			});
 		}
+
+		editor.formatter.register({
+			mark: { inline: 'mark' }
+		});
 	});
 
 	editor.on( 'SaveContent', function( event ) {