diff --git a/src/wp-includes/class-wp-query.php b/src/wp-includes/class-wp-query.php
index 437c82f1dd7f1..f9c7f090bc753 100644
--- a/src/wp-includes/class-wp-query.php
+++ b/src/wp-includes/class-wp-query.php
@@ -410,6 +410,13 @@ class WP_Query {
 	 */
 	public $is_favicon = false;
 
+	/**
+	 * Signifies whether the current query is for a sitemap.
+	 *
+	 * @since 7.1.0
+	 */
+	public bool $is_sitemap = false;
+
 	/**
 	 * Signifies whether the current query is for the page_for_posts page.
 	 *
@@ -519,6 +526,7 @@ private function init_query_flags() {
 		$this->is_singular          = false;
 		$this->is_robots            = false;
 		$this->is_favicon           = false;
+		$this->is_sitemap           = false;
 		$this->is_posts_page        = false;
 		$this->is_post_type_archive = false;
 	}
@@ -817,6 +825,8 @@ public function parse_query( $query = '' ) {
 			$this->is_robots = true;
 		} elseif ( ! empty( $query_vars['favicon'] ) ) {
 			$this->is_favicon = true;
+		} elseif ( ! empty( $query_vars['sitemap'] ) ) {
+			$this->is_sitemap = true;
 		}
 
 		if ( ! is_scalar( $query_vars['p'] ) || (int) $query_vars['p'] < 0 ) {
@@ -1040,7 +1050,7 @@ public function parse_query( $query = '' ) {
 
 		if ( ! ( $this->is_singular || $this->is_archive || $this->is_search || $this->is_feed
 				|| ( wp_is_serving_rest_request() && $this->is_main_query() )
-				|| $this->is_trackback || $this->is_404 || $this->is_admin || $this->is_robots || $this->is_favicon ) ) {
+				|| $this->is_trackback || $this->is_404 || $this->is_admin || $this->is_robots || $this->is_favicon || $this->is_sitemap ) ) {
 			$this->is_home = true;
 		}
 
@@ -4638,6 +4648,17 @@ public function is_favicon() {
 		return (bool) $this->is_favicon;
 	}
 
+	/**
+	 * Determines whether the query is for a sitemap.
+	 *
+	 * @since 7.1.0
+	 *
+	 * @return bool Whether the query is for a sitemap.
+	 */
+	public function is_sitemap(): bool {
+		return $this->is_sitemap;
+	}
+
 	/**
 	 * Determines whether the query is for a search.
 	 *
diff --git a/src/wp-includes/query.php b/src/wp-includes/query.php
index 592e70e0290a3..60571c01cb880 100644
--- a/src/wp-includes/query.php
+++ b/src/wp-includes/query.php
@@ -680,6 +680,26 @@ function is_favicon() {
 	return $wp_query->is_favicon();
 }
 
+/**
+ * Is the query for a sitemap?
+ *
+ * @since 7.1.0
+ *
+ * @global WP_Query $wp_query WordPress Query object.
+ *
+ * @return bool Whether the query is for a sitemap.
+ */
+function is_sitemap(): bool {
+	global $wp_query;
+
+	if ( ! isset( $wp_query ) ) {
+		_doing_it_wrong( __FUNCTION__, __( 'Conditional query tags do not work before the query is run. Before then, they always return false.' ), '7.1.0' );
+		return false;
+	}
+
+	return $wp_query->is_sitemap();
+}
+
 /**
  * Determines whether the query is for a search.
  *
diff --git a/tests/phpunit/includes/abstract-testcase.php b/tests/phpunit/includes/abstract-testcase.php
index b8e8598362ec5..55a9924fb23c3 100644
--- a/tests/phpunit/includes/abstract-testcase.php
+++ b/tests/phpunit/includes/abstract-testcase.php
@@ -1187,6 +1187,7 @@ public function assertQueryTrue( ...$prop ) {
 			'is_preview',
 			'is_robots',
 			'is_favicon',
+			'is_sitemap',
 			'is_search',
 			'is_single',
 			'is_singular',
diff --git a/tests/phpunit/tests/query/isSitemap.php b/tests/phpunit/tests/query/isSitemap.php
new file mode 100644
index 0000000000000..004aa1ed86455
--- /dev/null
+++ b/tests/phpunit/tests/query/isSitemap.php
@@ -0,0 +1,213 @@
+<?php
+
+/**
+ * Tests for the is_sitemap() conditional tag and the WP_Query::$is_sitemap property.
+ *
+ * This exercises both query.php and class-wp-query.php: query vars are fed through
+ * WP_Query, then the effects on the wp_query object are tested.
+ *
+ * @group query
+ * @group sitemaps
+ *
+ * @ticket 51543
+ */
+class Tests_Query_IsSitemap extends WP_UnitTestCase {
+
+	/**
+	 * Post IDs created for the shared fixture.
+	 *
+	 * @var int[]
+	 */
+	public static $post_ids;
+
+	/**
+	 * Set up the shared fixture.
+	 *
+	 * Published posts are required so that a sitemap request does not turn into a
+	 * 404 in WP::handle_404(), which would reset the is_sitemap flag via set_404().
+	 *
+	 * @param WP_UnitTest_Factory $factory Factory instance.
+	 */
+	public static function wpSetUpBeforeClass( WP_UnitTest_Factory $factory ) {
+		self::$post_ids = $factory->post->create_many( 3 );
+	}
+
+	public function set_up() {
+		parent::set_up();
+
+		$this->set_permalink_structure( '/%year%/%monthnum%/%day%/%postname%/' );
+
+		create_initial_taxonomies();
+	}
+
+	/**
+	 * The property defaults to false on a freshly initialized query.
+	 *
+	 * @covers WP_Query::init
+	 */
+	public function test_is_sitemap_defaults_to_false() {
+		$query = new WP_Query();
+
+		$this->assertFalse( $query->is_sitemap, 'The $is_sitemap property should default to false.' );
+		$this->assertFalse( $query->is_sitemap(), 'WP_Query::is_sitemap() should return false by default.' );
+	}
+
+	/**
+	 * The flag is set when the "sitemap" query var is present (sitemap index route).
+	 *
+	 * @covers WP_Query::parse_query
+	 * @covers WP_Query::is_sitemap
+	 */
+	public function test_is_sitemap_true_for_sitemap_index() {
+		$query = new WP_Query( array( 'sitemap' => 'index' ) );
+
+		$this->assertTrue( $query->is_sitemap, 'The $is_sitemap property should be true for a sitemap query.' );
+		$this->assertTrue( $query->is_sitemap(), 'WP_Query::is_sitemap() should return true for a sitemap query.' );
+	}
+
+	/**
+	 * The flag is set for a sitemap subtype route (e.g. wp-sitemap-posts-post-1.xml).
+	 *
+	 * @covers WP_Query::parse_query
+	 * @covers WP_Query::is_sitemap
+	 */
+	public function test_is_sitemap_true_for_sitemap_subtype() {
+		$query = new WP_Query(
+			array(
+				'sitemap'         => 'posts',
+				'sitemap-subtype' => 'post',
+				'paged'           => 1,
+			)
+		);
+
+		$this->assertTrue( $query->is_sitemap(), 'WP_Query::is_sitemap() should return true for a sitemap subtype query.' );
+	}
+
+	/**
+	 * is_sitemap() must return a boolean.
+	 *
+	 * @covers WP_Query::is_sitemap
+	 */
+	public function test_is_sitemap_return_type_is_bool() {
+		$query = new WP_Query( array( 'sitemap' => 'index' ) );
+
+		$this->assertIsBool( $query->is_sitemap(), 'WP_Query::is_sitemap() should return a boolean.' );
+	}
+
+	/**
+	 * An empty "sitemap" query var must not set the flag.
+	 *
+	 * @covers WP_Query::parse_query
+	 */
+	public function test_is_sitemap_false_for_empty_sitemap_var() {
+		$query = new WP_Query( array( 'sitemap' => '' ) );
+
+		$this->assertFalse( $query->is_sitemap(), 'An empty "sitemap" query var should not set is_sitemap.' );
+	}
+
+	/**
+	 * The sitemap stylesheet route uses the "sitemap-stylesheet" query var, which must
+	 * not flag the query as a sitemap.
+	 *
+	 * @covers WP_Query::parse_query
+	 */
+	public function test_is_sitemap_false_for_stylesheet_route() {
+		$query = new WP_Query( array( 'sitemap-stylesheet' => 'sitemap' ) );
+
+		$this->assertFalse( $query->is_sitemap(), 'The sitemap stylesheet route should not flag the query as a sitemap.' );
+	}
+
+	/**
+	 * is_robots takes precedence over is_sitemap in the parse_query branch.
+	 *
+	 * @covers WP_Query::parse_query
+	 */
+	public function test_robots_takes_precedence_over_sitemap() {
+		$query = new WP_Query(
+			array(
+				'robots'  => true,
+				'sitemap' => 'index',
+			)
+		);
+
+		$this->assertTrue( $query->is_robots(), 'is_robots() should be true when the robots query var is set.' );
+		$this->assertFalse( $query->is_sitemap(), 'is_sitemap() should be false when is_robots() takes precedence.' );
+	}
+
+	/**
+	 * A regular query is never flagged as a sitemap.
+	 *
+	 * @covers WP_Query::is_sitemap
+	 */
+	public function test_is_sitemap_false_for_regular_query() {
+		$post_id = self::factory()->post->create();
+
+		$query = new WP_Query( array( 'p' => $post_id ) );
+
+		$this->assertFalse( $query->is_sitemap(), 'A regular post query should not be flagged as a sitemap.' );
+	}
+
+	/**
+	 * A sitemap query must not also be treated as the home/front page.
+	 *
+	 * This is the practical motivation for the conditional tag: distinguishing a
+	 * sitemap request from the home page (see #51542).
+	 *
+	 * @covers WP_Query::parse_query
+	 */
+	public function test_sitemap_query_is_not_home() {
+		$query = new WP_Query( array( 'sitemap' => 'index' ) );
+
+		$this->assertTrue( $query->is_sitemap(), 'The sitemap query should be flagged as a sitemap.' );
+		$this->assertFalse( $query->is_home(), 'A sitemap query should not be treated as the home page.' );
+		$this->assertFalse( $query->is_front_page(), 'A sitemap query should not be treated as the front page.' );
+	}
+
+	/**
+	 * The global is_sitemap() conditional tag reflects the main query.
+	 *
+	 * @covers ::is_sitemap
+	 */
+	public function test_global_is_sitemap_reflects_main_query() {
+		// Prevent WP_Sitemaps from rendering and calling exit during go_to().
+		remove_action( 'template_redirect', array( wp_sitemaps_get_server(), 'render_sitemaps' ) );
+
+		$this->go_to( home_url( '/?sitemap=index' ) );
+
+		$this->assertTrue( is_sitemap(), 'is_sitemap() should be true on a sitemap request.' );
+
+		// is_sitemap should be the only conditional that is true for a sitemap request.
+		$this->assertQueryTrue( 'is_sitemap' );
+	}
+
+	/**
+	 * The global is_sitemap() conditional tag is false for a non-sitemap request.
+	 *
+	 * @covers ::is_sitemap
+	 */
+	public function test_global_is_sitemap_false_on_home() {
+		$this->go_to( home_url( '/' ) );
+
+		$this->assertFalse( is_sitemap(), 'is_sitemap() should be false on the home page.' );
+		$this->assertTrue( is_home(), 'is_home() should be true on the home page.' );
+	}
+
+	/**
+	 * The global is_sitemap() returns false and triggers a notice when the query
+	 * has not yet run.
+	 *
+	 * @covers ::is_sitemap
+	 *
+	 * @expectedIncorrectUsage is_sitemap
+	 */
+	public function test_global_is_sitemap_before_query_is_run() {
+		$wp_query_temp = $GLOBALS['wp_query'];
+		unset( $GLOBALS['wp_query'] );
+
+		$result = is_sitemap();
+
+		$GLOBALS['wp_query'] = $wp_query_temp;
+
+		$this->assertFalse( $result, 'is_sitemap() should return false before the query is run.' );
+	}
+}