diff --git a/src/wp-includes/interactivity-api/class-wp-interactivity-api.php b/src/wp-includes/interactivity-api/class-wp-interactivity-api.php
index bc8da9c9b2e7b..506dc4a518b71 100644
--- a/src/wp-includes/interactivity-api/class-wp-interactivity-api.php
+++ b/src/wp-includes/interactivity-api/class-wp-interactivity-api.php
@@ -1314,6 +1314,14 @@ private function data_wp_router_region_processor( WP_Interactivity_API_Directive
 		if ( 'enter' === $mode && ! $this->has_processed_router_region ) {
 			$this->has_processed_router_region = true;
 
+			// Initializes the `state.url` property from the server.
+			$this->state(
+				'core/router',
+				array(
+					'url' => get_self_link(),
+				)
+			);
+
 			// Enqueues as an inline style.
 			wp_register_style( 'wp-interactivity-router-animations', false );
 			wp_add_inline_style( 'wp-interactivity-router-animations', $this->get_router_animation_styles() );
diff --git a/tests/phpunit/tests/interactivity-api/wpInteractivityAPI-wp-router-region.php b/tests/phpunit/tests/interactivity-api/wpInteractivityAPI-wp-router-region.php
index f5e5e3a62d37c..ee31459855658 100644
--- a/tests/phpunit/tests/interactivity-api/wpInteractivityAPI-wp-router-region.php
+++ b/tests/phpunit/tests/interactivity-api/wpInteractivityAPI-wp-router-region.php
@@ -76,6 +76,25 @@ protected function render_wp_footer() {
 		return ob_get_clean();
 	}
 
+	/**
+	 * Processes directives while temporarily replacing the global
+	 * WP_Interactivity_API instance so that global functions like
+	 * `wp_interactivity_state` operate on the test instance.
+	 *
+	 * @param string $html The HTML to process.
+	 * @return string The processed HTML.
+	 */
+	protected function process_directives( string $html ): string {
+		global $wp_interactivity;
+		$prev             = $wp_interactivity;
+		$wp_interactivity = $this->interactivity;
+
+		$result = $this->interactivity->process_directives( $html );
+
+		$wp_interactivity = $prev;
+		return $result;
+	}
+
 	/**
 	 * Tests that no elements are added if the `data-wp-router-region` is
 	 * missing.
@@ -126,4 +145,58 @@ public function test_wp_router_region_adds_loading_bar_region_only_once() {
 		$this->assertTrue( $p->next_tag( $query ) );
 		$this->assertFalse( $p->next_tag( $query ) );
 	}
+
+	/**
+	 * Tests that the `data-wp-router-region` directive initializes the
+	 * `core/router` state URL from the server.
+	 *
+	 * @ticket 64649
+	 *
+	 * @covers ::process_directives
+	 */
+	public function test_wp_router_region_initializes_state_url() {
+		$_SERVER['REQUEST_URI'] = '/test-page/?query=1';
+
+		$html = '<div data-wp-router-region="region A">Interactive region</div>';
+		$this->process_directives( $html );
+
+		$state = $this->interactivity->state( 'core/router' );
+		$this->assertSame( home_url( '/test-page/?query=1' ), $state['url'] );
+	}
+
+	/**
+	 * Tests that the `core/router` state URL uses HTTPS when SSL is active.
+	 *
+	 * @ticket 64649
+	 *
+	 * @covers ::process_directives
+	 */
+	public function test_wp_router_region_initializes_state_url_with_https() {
+		$_SERVER['REQUEST_URI'] = '/';
+		$_SERVER['HTTPS']       = 'on';
+
+		$html = '<div data-wp-router-region="region A">Interactive region</div>';
+		$this->process_directives( $html );
+
+		$state = $this->interactivity->state( 'core/router' );
+		$this->assertStringStartsWith( 'https://', $state['url'] );
+	}
+
+	/**
+	 * Tests that the `core/router` state URL is not set when no
+	 * `data-wp-router-region` directive is present.
+	 *
+	 * @ticket 64649
+	 *
+	 * @covers ::process_directives
+	 */
+	public function test_wp_router_region_does_not_set_state_url_without_directive() {
+		$_SERVER['REQUEST_URI'] = '/';
+
+		$html = '<div>Nothing here</div>';
+		$this->process_directives( $html );
+
+		$state = $this->interactivity->state( 'core/router' );
+		$this->assertEmpty( $state );
+	}
 }