Facilitating single sign-on (SSO) across multiple browser instances such that user authentication at one browser instance is used as a basis to permit access to protected resources (hosted on server systems) from other browser instances. In an embodiment, the different browser instances are executing on different client systems. An authentication server may maintain a registration data indicating the different client systems/browser instances registered by a user for SSO feature. After a user is authenticated for a first session from one browser instance, the authentication server enables the user to access any protected resource from registered client systems/browser instances without requiring further authentication (based on the presence of the authenticated first session).