diff --git a/src/wp-admin/css/common.css b/src/wp-admin/css/common.css
index d91e8a1368..2d64bca75e 100644
--- a/src/wp-admin/css/common.css
+++ b/src/wp-admin/css/common.css
@@ -3147,6 +3147,10 @@ img {
 	line-height: 180%;
 }
 
+#file-editor-warning .file-editor-warning-content {
+	margin: 25px;
+}
+
 /* @todo: can we use a common class for these? */
 .nav-menus-php .item-edit:before,
 .widget-top .widget-action .toggle-indicator:before,
diff --git a/src/wp-admin/js/theme-plugin-editor.js b/src/wp-admin/js/theme-plugin-editor.js
index 3bb0788a6b..24c969fe55 100644
--- a/src/wp-admin/js/theme-plugin-editor.js
+++ b/src/wp-admin/js/theme-plugin-editor.js
@@ -46,6 +46,14 @@ wp.themePluginEditor = (function( $ ) {
 		component.form.on( 'submit', component.submit );
 		component.textarea = component.form.find( '#newcontent' );
 		component.textarea.on( 'change', component.onChange );
+		component.warning = $( '.file-editor-warning' );
+
+		if ( component.warning.length > 0 ) {
+			$( 'body' ).addClass( 'modal-open' );
+			component.warning.find( '.file-editor-warning-dismiss' ).focus();
+			component.warning.on( 'click', '.file-editor-warning-dismiss', component.dismissWarning );
+		};
+
 
 		if ( false !== component.codeEditor ) {
 			/*
@@ -66,6 +74,26 @@ wp.themePluginEditor = (function( $ ) {
 		} );
 	};
 
+	/**
+	 * Dismiss the warning modal.
+	 *
+	 * @since 4.9.0
+	 * @returns {void}
+	 */
+	component.dismissWarning = function() {
+
+		wp.ajax.post( 'dismiss-wp-pointer', {
+			pointer: component.themeOrPlugin + '_editor_notice'
+		});
+
+		// hide modal
+		component.warning.remove();
+		$( 'body' ).removeClass( 'modal-open' );
+
+		// return focus - is this a trap?
+		component.instance.codemirror.focus();
+	};
+
 	/**
 	 * Callback for when a change happens.
 	 *
diff --git a/src/wp-admin/plugin-editor.php b/src/wp-admin/plugin-editor.php
index 636e1cf739..807cd5e09d 100644
--- a/src/wp-admin/plugin-editor.php
+++ b/src/wp-admin/plugin-editor.php
@@ -142,6 +142,7 @@ if ( 'POST' === $_SERVER['REQUEST_METHOD'] ) {
 	);
 	wp_enqueue_script( 'wp-theme-plugin-editor' );
 	wp_add_inline_script( 'wp-theme-plugin-editor', sprintf( 'jQuery( function( $ ) { wp.themePluginEditor.init( $( "#template" ), %s ); } )', wp_json_encode( $settings ) ) );
+	wp_add_inline_script( 'wp-theme-plugin-editor', sprintf( 'wp.themePluginEditor.themeOrPlugin = "plugin";' ) );
 
 	require_once(ABSPATH . 'wp-admin/admin-header.php');
 
@@ -280,5 +281,21 @@ if ( 'POST' === $_SERVER['REQUEST_METHOD'] ) {
 <br class="clear" />
 </div>
 <?php
+$dismissed_pointers = explode( ',', (string) get_user_meta( get_current_user_id(), 'dismissed_wp_pointers', true ) );
+if ( ! in_array( 'plugin_editor_notice', $dismissed_pointers, true ) ) :
+?>
+<div id="file-editor-warning" class="notification-dialog-wrap file-editor-warning hide-if-no-js">
+	<div class="notification-dialog-background"></div>
+	<div class="notification-dialog" role="dialog" aria-labelledby="file-editor-warning-title" tabindex="0">
+		<div class="file-editor-warning-content">
+			<h1 id="file-editor-warning-title"><?php _e( 'Heads up!' ); ?></h1>
+			<p><?php _e( 'You appear to be making direct edits to your plugin in the WordPress dashboard. We recommend that you don&#8217;t! Editing plugins directly may introduce incompatibilities that break your theme or other plugins, and can leave you unable to log back in to WordPress and undo changes.' ); ?></p>
+			<p><?php _e( 'If you absolutely have to edit this plugin, create a copy with a new name and hang on to the original version, so you can re-enable a functional version if something goes wrong.' ); ?></p>
+			<p><button type="button" class="file-editor-warning-dismiss button-primary"><?php _e( 'I understand' ); ?></button></p>
+		</div>
+	</div>
+</div>
+<?php
+endif; // editor warning notice
 
 include(ABSPATH . "wp-admin/admin-footer.php");
diff --git a/src/wp-admin/theme-editor.php b/src/wp-admin/theme-editor.php
index b49013ffd8..ebac61b538 100644
--- a/src/wp-admin/theme-editor.php
+++ b/src/wp-admin/theme-editor.php
@@ -127,6 +127,7 @@ if ( 'POST' === $_SERVER['REQUEST_METHOD'] ) {
 	);
 	wp_enqueue_script( 'wp-theme-plugin-editor' );
 	wp_add_inline_script( 'wp-theme-plugin-editor', sprintf( 'jQuery( function( $ ) { wp.themePluginEditor.init( $( "#template" ), %s ); } )', wp_json_encode( $settings ) ) );
+	wp_add_inline_script( 'wp-theme-plugin-editor', 'wp.themePluginEditor.themeOrPlugin = "theme";' );
 
 	require_once( ABSPATH . 'wp-admin/admin-header.php' );
 
@@ -309,5 +310,29 @@ endif; // $error
 <br class="clear" />
 </div>
 <?php
+$dismissed_pointers = explode( ',', (string) get_user_meta( get_current_user_id(), 'dismissed_wp_pointers', true ) );
+if ( ! in_array( 'theme_editor_notice', $dismissed_pointers, true ) ) :
+?>
+<div id="file-editor-warning" class="notification-dialog-wrap file-editor-warning hide-if-no-js">
+	<div class="notification-dialog-background"></div>
+	<div class="notification-dialog" role="dialog" aria-labelledby="file-editor-warning-title" tabindex="0">
+		<div class="file-editor-warning-content">
+			<h1 id="file-editor-warning-title"><?php _e( 'Heads up!' ); ?></h1>
+			<p>
+				<?php
+				echo sprintf(
+					/* translators: %s is a link to Custom CSS section in the Customizer. */
+					__( 'You appear to be making direct edits to your theme in the WordPress Dashboard. We recommend that you don&#8217;t! Editing this code directly is dangerous, and can leave you unable to log back in to WordPress and undo changes. There&#8217;s no need to change your CSS here &mdash; you can edit and live preview CSS changes in WordPress&#8217;s <a href="%s">built in CSS editor</a>.' ),
+					esc_url( add_query_arg( 'autofocus[section]', 'custom_css', admin_url( 'customize.php' ) ) )
+				);
+				?>
+			</p>
+			<p><?php _e( 'If you decide to go ahead with direct edits anyway, make sure to back up all your site&#8217;s files before making changes so you can restore a functional version if something goes wrong.' ); ?></p>
+			<p><button type="button" class="file-editor-warning-dismiss button-primary"><?php _e( 'I understand' ); ?></button></p>
+		</div>
+	</div>
+</div>
+<?php
+endif; // editor warning notice
 
 include(ABSPATH . 'wp-admin/admin-footer.php' );