How to add a vertical dropdown menu in WordPress sidebar

To add a custom vertical menu, the first thing you need to do is register your new navigation menu by adding this code to your theme’s functions.php file.

function register_my_menu() {
  register_nav_menu('navigation',__( 'Sidebar Menu' ));
}
add_action( 'init', 'register_my_menu' );

You will need to add this code in your theme’s template file where you want to display your menu. I added in the sidebar section of header.php

<?php wp_nav_menu( array( 'theme_location' => 'navigation' ) ); ?>

Add the following in style.css file

.menu-menu-1-container ul,
.menu-menu-1-container ul li,
.menu-menu-1-container ul ul {
  list-style: none;
  margin: 0;
  padding: 0;
}

.menu-menu-1-container ul {
  position: relative;
  z-index: 500;
}

.menu-menu-1-container ul li {
  float: left;
  min-height: 0.05em;
  line-height: 1em;
  vertical-align: middle;
  position: relative;
}

.menu-menu-1-container ul li.hover,
.menu-menu-1-container ul li:hover {
  position: relative;
  z-index: 510;
  cursor: default;
}

.menu-menu-1-container ul ul {
  visibility: hidden;
  position: absolute;
  top: 100%;
  left: 0px;
  z-index: 520;
  width: 100%;
}

.menu-menu-1-container ul ul li { float: none; }

.menu-menu-1-container ul ul ul {
  top: 0;
  right: 0;
}

.menu-menu-1-container ul li:hover > ul { visibility: visible; }

.menu-menu-1-container ul ul {
  top: 0;
  left: 99%;
}

.menu-menu-1-container ul li { float: none; }

.menu-menu-1-container ul ul { margin-top: 0.05em; }

.menu-menu-1-container {
  zoom: 1;
}

.menu-menu-1-container:before {
  content: '';
  display: block;
}

.menu-menu-1-container:after {
  content: '';
  display: table;
  clear: both;
}

.menu-menu-1-container a {
  display: block;
  padding: 1em 1.3em;
  color: #ffffff;
  text-decoration: none;
  text-transform: uppercase;
}

.menu-menu-1-container > ul > li > a {
  color: #ffffff;
}

.menu-menu-1-container > ul > li > a:hover { color: #ffffff; }

.menu-menu-1-container > ul > li a:hover,
.menu-menu-1-container > ul > li:hover a { background: #34A65F; }

.menu-menu-1-container li { position: relative; }

.menu-menu-1-container ul li.sub-menu-menu-1-container > a:after {
  content: 'Â»';
  position: absolute;
  right: 1em;
}

.menu-menu-1-container ul ul li.first {
  -webkit-border-radius: 0 3px 0 0;
  -moz-border-radius: 0 3px 0 0;
  border-radius: 0 3px 0 0;
}

.menu-menu-1-container ul ul li.last {
  -webkit-border-radius: 0 0 3px 0;
  -moz-border-radius: 0 0 3px 0;
  border-radius: 0 0 3px 0;
  border-bottom: 0;
}

.menu-menu-1-container ul ul {
  -webkit-border-radius: 0 3px 3px 0;
  -moz-border-radius: 0 3px 3px 0;
  border-radius: 0 3px 3px 0;
}

.menu-menu-1-container ul ul { border: 1px solid #34A65F; }

.menu-menu-1-container ul ul a { color: #ffffff; }

.menu-menu-1-container ul ul a:hover { color: #ffffff; }

.menu-menu-1-container ul ul li { border-bottom: 1px solid #0F8A5F; }

.menu-menu-1-container ul ul li:hover > a {
  background: #4eb1ff;
  color: #ffffff;
}

.menu-menu-1-container.align-right > ul > li > a {
  border-left: 0.3em solid #34A65F;
  border-right: none;
}

.menu-menu-1-container.align-right { float: right; }

.menu-menu-1-container.align-right li { text-align: right; }

.menu-menu-1-container.align-right ul li.sub-menu-menu-1-container > a:before {
  content: '+';
  position: absolute;
  top: 50%;
  left: 15px;
  margin-top: -6px;
}

.menu-menu-1-container.align-right ul li.sub-menu-menu-1-container > a:after { content: none; }

.menu-menu-1-container.align-right ul ul {
  visibility: hidden;
  position: absolute;
  top: 0;
  left: -100%;
  z-index: 598;
  width: 100%;
}

.menu-menu-1-container.align-right ul ul li.first {
  -webkit-border-radius: 3px 0 0 0;
  -moz-border-radius: 3px 0 0 0;
  border-radius: 3px 0 0 0;
}

.menu-menu-1-container.align-right ul ul li.last {
  -webkit-border-radius: 0 0 0 3px;
  -moz-border-radius: 0 0 0 3px;
  border-radius: 0 0 0 3px;
}

.menu-menu-1-container.align-right ul ul {
  -webkit-border-radius: 3px 0 0 3px;
  -moz-border-radius: 3px 0 0 3px;
  border-radius: 3px 0 0 3px;
}

Now go to Admin > Appearance > Menus page in your WordPress admin and try to create or edit a new menu. You can see ‘Sidebar Menu‘ as theme location option.