By default the Sitemap module of the Jetpack plugin for WordPress does not include custom post types in its generated sitemap.xml.

Below are three methods for adding this functionality to your WordPress website.

Method 1 - Add to an existing array

This site uses a custom portfolio post-type that is created in the theme’s functions.php. Here’s how it looks:

//* Create portfolio custom post type
add_action( 'init', 'bayareawebs_portfolio_post_type' );
function bayareawebs_portfolio_post_type() {

	register_post_type( 'portfolio',
		array(
			'labels' => array(
				'name'          => __( 'Portfolio', 'bayareawebs' ),
				'singular_name' => __( 'Portfolio', 'bayareawebs' ),
			),
			'has_archive'  => true,
			'hierarchical' => true,
			'menu_icon'    => get_stylesheet_directory_uri() . '/lib/icons/portfolio.png',
			'public'       => true,
			'rewrite'      => array( 'slug' => 'portfolio', 'with_front' => false ),
			'supports'     => array( 'title', 'editor', 'author', 'thumbnail', 'excerpt', 'trackbacks', 'custom-fields', 'revisions', 'page-attributes', 'genesis-seo', 'genesis-cpt-archives-settings' ),
			'taxonomies'   => array( 'portfolio-type' ),

		)
	);

}

Inside 'supports', simply add 'jetpack_sitemap_post_types'. The end result looks like this:

'supports' => array( 'title', 'editor', 'author', 'thumbnail', 'excerpt', 'trackbacks', 'custom-fields', 'revisions', 'page-attributes', 'genesis-seo', 'genesis-cpt-archives-settings', 'jetpack_sitemap_post_types'

Method 2 - Add a filter in functions.php

This method is a bit cleaner and is copied from this page in the WordPress forums. Simply copy and paste this into your functions.php and replace your_post_type with the post type you want to add.

function jeherve_add_cpt_sitemaps( $post_types ) {
	$post_types[] = 'your_post_type';
	return $post_types;
}
add_filter( 'jetpack_sitemap_post_types', 'jeherve_add_cpt_sitemaps' );

Method 3 - Edit the Jetpack plugin directly

Not recommended, since a plugin update will overwrite it. You can head to /wp-content/plugins/jetpack/modules/sitemaps/sitemap.php and add an extra post type around line 268, so it looks something like this:

$post_types    = apply_filters( 'jetpack_sitemap_post_types', array( 'post', 'page', 'portfolio' ) );

Last step

Publish and delete a post or page. That will trigger the cron job that regenerates the sitemap.