diff --git a/tests/phpunit/tests/load/wpDoingSitemap.php b/tests/phpunit/tests/load/wpDoingSitemap.php
new file mode 100644
index 0000000000000..a1ce10496575c
--- /dev/null
+++ b/tests/phpunit/tests/load/wpDoingSitemap.php
@@ -0,0 +1,67 @@
+<?php
+
+/**
+ * Tests for wp_doing_sitemap().
+ *
+ * @group load
+ * @group sitemaps
+ *
+ * @covers ::wp_doing_sitemap
+ */
+class Tests_Load_WpDoingSitemap extends WP_UnitTestCase {
+
+	/**
+	 * The function should return false on a regular request.
+	 *
+	 * @ticket 56954
+	 */
+	public function test_should_return_false_by_default() {
+		$this->assertFalse( wp_doing_sitemap() );
+	}
+
+	/**
+	 * The 'wp_doing_sitemap' filter should be able to force a true result.
+	 *
+	 * @ticket 56954
+	 */
+	public function test_filter_can_force_true() {
+		add_filter( 'wp_doing_sitemap', '__return_true' );
+
+		$this->assertTrue( wp_doing_sitemap() );
+	}
+
+	/**
+	 * The DOING_SITEMAP constant should make the function return true.
+	 *
+	 * Runs in a separate process because the constant cannot be undefined
+	 * once it has been set.
+	 *
+	 * @ticket 56954
+	 *
+	 * @runInSeparateProcess
+	 * @preserveGlobalState disabled
+	 */
+	public function test_constant_defined_returns_true() {
+		$this->assertFalse( wp_doing_sitemap(), 'wp_doing_sitemap() should be false before the constant is defined.' );
+
+		define( 'DOING_SITEMAP', true );
+
+		$this->assertTrue( wp_doing_sitemap(), 'wp_doing_sitemap() should be true once DOING_SITEMAP is defined.' );
+	}
+
+	/**
+	 * The 'wp_doing_sitemap' filter should be able to override the constant.
+	 *
+	 * @ticket 56954
+	 *
+	 * @runInSeparateProcess
+	 * @preserveGlobalState disabled
+	 */
+	public function test_filter_overrides_constant() {
+		define( 'DOING_SITEMAP', true );
+
+		add_filter( 'wp_doing_sitemap', '__return_false' );
+
+		$this->assertFalse( wp_doing_sitemap() );
+	}
+}
diff --git a/tests/phpunit/tests/sitemaps/sitemaps.php b/tests/phpunit/tests/sitemaps/sitemaps.php
index 85f9965245842..e24c2d9bc7282 100644
--- a/tests/phpunit/tests/sitemaps/sitemaps.php
+++ b/tests/phpunit/tests/sitemaps/sitemaps.php
@@ -493,4 +493,52 @@ public function test_empty_url_list_should_return_404() {
 
 		$this->assertTrue( is_404() );
 	}
+
+	/**
+	 * Ensures rendering a sitemap defines the DOING_SITEMAP constant.
+	 *
+	 * @ticket 56954
+	 *
+	 * @covers WP_Sitemaps::render_sitemaps
+	 * @covers ::wp_doing_sitemap
+	 *
+	 * @runInSeparateProcess
+	 * @preserveGlobalState disabled
+	 */
+	public function test_render_sitemaps_defines_doing_sitemap() {
+		$this->assertFalse( wp_doing_sitemap(), 'wp_doing_sitemap() should be false before a sitemap is rendered.' );
+
+		wp_register_sitemap_provider( 'foo', new WP_Sitemaps_Empty_Test_Provider( 'foo' ) );
+
+		$this->go_to( home_url( '/?sitemap=foo' ) );
+
+		wp_sitemaps_get_server()->render_sitemaps();
+
+		$this->assertTrue( defined( 'DOING_SITEMAP' ), 'The DOING_SITEMAP constant should be defined.' );
+		$this->assertTrue( wp_doing_sitemap(), 'wp_doing_sitemap() should be true while rendering a sitemap.' );
+	}
+
+	/**
+	 * Ensures the DOING_SITEMAP constant is not defined when sitemaps are disabled.
+	 *
+	 * The constant is only defined once the request has passed the preliminary
+	 * checks, so a disabled sitemap request should never reach that point.
+	 *
+	 * @ticket 56954
+	 *
+	 * @covers WP_Sitemaps::render_sitemaps
+	 *
+	 * @runInSeparateProcess
+	 * @preserveGlobalState disabled
+	 */
+	public function test_render_sitemaps_does_not_define_doing_sitemap_when_disabled() {
+		add_filter( 'wp_sitemaps_enabled', '__return_false' );
+
+		$this->go_to( home_url( '/?sitemap=index' ) );
+
+		wp_sitemaps_get_server()->render_sitemaps();
+
+		$this->assertFalse( defined( 'DOING_SITEMAP' ), 'The DOING_SITEMAP constant should not be defined when sitemaps are disabled.' );
+		$this->assertFalse( wp_doing_sitemap(), 'wp_doing_sitemap() should be false when sitemaps are disabled.' );
+	}
 }