How to add one section separator for Navigation Drawer in Android?

Make sure you define each group with a unique ID, separator won't appear without the ID.

For example, this is my drawer_menu.xml:

<?xml version="1.0" encoding="utf-8"?><menu xmlns:android="http://schemas.android.com/apk/res/android">    <group        android:id="@+id/menu_top"        android:checkableBehavior="single">        <item            android:checked="true"            android:id="@+id/drawer_item_timeline"            android:icon="@drawable/ic_timer_grey600_24dp"            android:title="@string/drawer_timeline"/>        <item            android:id="@+id/drawer_item_reports"            android:icon="@drawable/ic_timetable_grey600_24dp"            android:title="@string/drawer_reports"/>    </group>    <group        android:id="@+id/menu_bottom"        android:checkableBehavior="none">        <item            android:id="@+id/drawer_item_settings"            android:icon="@drawable/ic_settings_black_24dp"            android:title="@string/drawer_settings" >        </item>    </group></menu>

Gabriel adds below in the comments that if the group doesn't have an id, the separator will not appear.

To separate menu items by a divider line, only group items with a unique id like following example:

activity_main_drawer.xml:

<?xml version="1.0" encoding="utf-8"?><menu xmlns:android="http://schemas.android.com/apk/res/android"    xmlns:tools="http://schemas.android.com/tools"    tools:showIn="navigation_view">    <item        android:id="@+id/nav_apps_and_games"        android:icon="@drawable/ic_apps_black_24dp"        android:title="@string/my_apps_and_games" />    <item        android:id="@+id/nav_bookmarked_apps"        android:icon="@drawable/ic_add_bookmark_black_24dp"        android:title="@string/bookmarked_apps" />    <item        android:id="@+id/nav_manage_downloads"        android:icon="@drawable/ic_downloading_file_black_24dp"        android:title="@string/manage_downloads" />    <!-- SET A UNIQUE ID TO THE BELOW GROUP -->    <group android:id="@+id/group1">        <item            android:id="@+id/nav_settings"            android:icon="@drawable/ic_settings_black_24dp"            android:title="@string/settings" />        <item            android:id="@+id/nav_sign_up"            android:icon="@drawable/ic_card_membership_black_24dp"            android:title="@string/sign_up_login" />    </group></menu>

Visual Result:

My hacky method is similar to Mostrapotski's.

In my Layout for my custom adapter, I'm adding a horizontal separator at the beginning of each item and setting it's visibility to gone.

For the elements that mark the beginning of a new group, I set their visibility to visible so that the separator shows up on top of it.