Snippet

if (\Drupal::service('router.admin_context')->isAdminRoute()) {
  // ...
}

The router.admin_context service maps to the AdminContext helper class.

isAdminRoute() simply checks the Route for the _admin_route option.

Pitfalls

When working with a Route object, it's tempting to check for the _admin_route option directly, instead of using the router_admin_context service. The problem here is any customization of the service will be ignored.

Relevant links

• Admin paths are now defined as part of route definitions: Drupal 7 to 8 change record, with a Matching admin routes section referring to this.
• Structure of routes: Information about the _admin_route option (and Drupal routes in general).
• How to check if the current page using admin theme in Drupal?: Similar blog post.