diff --git a/src/wp-admin/css/colors-classic.css b/src/wp-admin/css/colors-classic.css
index 32bc6f4bf4..c481fdc621 100644
--- a/src/wp-admin/css/colors-classic.css
+++ b/src/wp-admin/css/colors-classic.css
@@ -734,6 +734,16 @@ table.widefat span.spam a,
 	color: #555;
 }
 
+.update-nag-core-insecure {
+	color: #fff;
+	background-color: #ac1b1b;
+	border-color: #cd5a5a;
+}
+
+.update-nag-core-insecure a {
+	color: #fff;
+}
+
 #screen-meta {
 	background-color: #eff8ff;
 	border-color: #d1e5ee;
diff --git a/src/wp-admin/css/colors-fresh.css b/src/wp-admin/css/colors-fresh.css
index 547d6f4173..2fd7f20be8 100644
--- a/src/wp-admin/css/colors-fresh.css
+++ b/src/wp-admin/css/colors-fresh.css
@@ -732,6 +732,16 @@ table.widefat span.spam a,
 	color: #555;
 }
 
+.update-nag-core-insecure {
+	color: #fff;
+	background-color: #ac1b1b;
+	border-color: #cd5a5a;
+}
+
+.update-nag-core-insecure a {
+	color: #fff;
+}
+
 #screen-meta {
 	background-color: #f1f1f1;
 	border-color: #ccc;
diff --git a/src/wp-admin/css/wp-admin.css b/src/wp-admin/css/wp-admin.css
index 2f5b5c7fcc..59ed455393 100644
--- a/src/wp-admin/css/wp-admin.css
+++ b/src/wp-admin/css/wp-admin.css
@@ -1123,6 +1123,31 @@ th.action-links {
 	border-bottom-left-radius: 3px;
 }
 
+.update-nag-core-insecure {
+	color: #fff;
+	background-color: #ac1b1b;
+	border-color: #cd5a5a;
+	font-weight: bold;
+}
+
+.update-nag-core-insecure > p {
+	margin-left: 1em;
+	margin-right: 1em;
+}
+
+.update-nag-core-insecure > p:first-child {
+	margin-top: 0;
+	clear: right; /* Screen options, help tabs */
+}
+
+.update-nag-core-insecure > p:last-child {
+	margin-bottom: 0;
+}
+
+.update-nag-core-insecure a {
+	color: #fff;
+}
+
 .plugins .plugin-update {
 	padding: 0;
 }
diff --git a/src/wp-admin/includes/update.php b/src/wp-admin/includes/update.php
index 5ee195a79f..c50e033d10 100644
--- a/src/wp-admin/includes/update.php
+++ b/src/wp-admin/includes/update.php
@@ -210,11 +210,23 @@ function update_nag() {
 		return false;
 
 	if ( current_user_can('update_core') ) {
-		$msg = sprintf( __('<a href="http://codex.wordpress.org/Version_%1$s">WordPress %1$s</a> is available! <a href="%2$s">Please update now</a>.'), $cur->current, network_admin_url( 'update-core.php' ) );
+		$msg       = sprintf( __('<a href="http://codex.wordpress.org/Version_%1$s">WordPress %1$s</a> is available! <a href="%2$s">Please update now</a>.'), $cur->current, network_admin_url( 'update-core.php' ) );
+		$msg_line2 = sprintf(
+			/* translators: 1: WordPress version number, 2: Link to update WordPress */
+			__( 'Important! Your version of WordPress (%1$s) is no longer supported, you will not receive any security updates for your website. To keep your site secure, please <a href="%2$s">update to the latest version of WordPress</a>.' ),
+			get_bloginfo( 'version', 'display' ),
+			network_admin_url( 'update-core.php' )
+		);
 	} else {
-		$msg = sprintf( __('<a href="http://codex.wordpress.org/Version_%1$s">WordPress %1$s</a> is available! Please notify the site administrator.'), $cur->current );
+		$msg       = sprintf( __('<a href="http://codex.wordpress.org/Version_%1$s">WordPress %1$s</a> is available! Please notify the site administrator.'), $cur->current );
+		$msg_line2 = sprintf(
+			/* translators: 1: WordPress version number, 2: Link to update WordPress */
+			__( 'Important! Your version of WordPress (%1$s) is no longer supported, you will not receive any security updates for your website. To keep your site secure, please <a href="%2$s">update to the latest version of WordPress</a>.' ),
+			get_bloginfo( 'version', 'display' ),
+			'https://wordpress.org/download/'
+		);
 	}
-	echo "<div class='update-nag'>$msg</div>";
+	echo "<div class='update-nag update-nag-core-insecure'><p>$msg</p><p>$msg_line2</p></div>";
 }
 add_action( 'admin_notices', 'update_nag', 3 );
 add_action( 'network_admin_notices', 'update_nag', 3 );