Add a Custom Post Type to the Category Page in WordPress

Jul 10, 2020 | WordPress

function custom_post_type_cat_filter($query) {
  if ( !is_admin() && $query->is_main_query() ) {
    if ($query->is_category()) {
      $query->set( 'post_type', array( 'post', 'xxx' ) );
    }
  }
}
add_action('pre_get_posts','custom_post_type_cat_filter');

Custom post types are excluded from the main query by default so you need to include the custom post type in the main query manually. This uses pre_get_posts which changes the main query before it is executed.