diff --git a/src/wp-includes/ms-functions.php b/src/wp-includes/ms-functions.php
index d1536eb030..a562cdf241 100644
--- a/src/wp-includes/ms-functions.php
+++ b/src/wp-includes/ms-functions.php
@@ -96,11 +96,13 @@ function get_active_blog_for_user( $user_id ) {
  * The count is cached and updated twice daily. This is not a live count.
  *
  * @since MU 2.7
+ * @since 4.8.0 The $network_id parameter has been added.
  *
- * @return int
+ * @param int|null $network_id ID of the network. Default is the current network.
+ * @return int Number of active users on the network.
  */
-function get_user_count() {
-	return get_site_option( 'user_count' );
+function get_user_count( $network_id = null ) {
+	return get_network_option( $network_id, 'user_count' );
 }
 
 /**
diff --git a/tests/phpunit/tests/multisite/network.php b/tests/phpunit/tests/multisite/network.php
index 6a94dafe44..ffd976e10c 100644
--- a/tests/phpunit/tests/multisite/network.php
+++ b/tests/phpunit/tests/multisite/network.php
@@ -205,6 +205,35 @@ class Tests_Multisite_Network extends WP_UnitTestCase {
 		$this->assertSame( count( self::$different_site_ids ), $site_count );
 	}
 
+	/**
+	 * @ticket 37866
+	 */
+	public function test_get_user_count_on_different_network() {
+		global $current_site, $wpdb;
+
+		wp_update_network_user_counts();
+		$current_network_user_count = get_user_count();
+
+		// switch_to_network()...
+		$orig_network_id = $current_site->id;
+		$orig_wpdb_network_id = $wpdb->siteid;
+		$current_site->id = self::$different_network_id;
+		$wpdb->siteid = self::$different_network_id;
+
+		// Add another user to fake the network user count to be different.
+		wpmu_create_user( 'user', 'pass', 'email' );
+
+		wp_update_network_user_counts();
+
+		// restore_current_network()...
+		$current_site->id = $orig_network_id;
+		$wpdb->siteid = $orig_wpdb_network_id;
+
+		$user_count = get_user_count( self::$different_network_id );
+
+		$this->assertEquals( $current_network_user_count + 1, $user_count );
+	}
+
 	/**
 	 * @ticket 22917
 	 */