A refrigerator includes a cabinet, a main door, an auxiliary storage compartment provided to a rear surface of the main door, and a sub-door. The main door is rotatably connected to the cabinet, and the sub-door is rotatably connected to the main door. An access opening is provided in the main door to allow access to the auxiliary storage compartment. The refrigerator also includes a button exposed at a front surface of the sub-door to receive input provided by a user, a hook member having a pivot shaft mounted to its rear end, and a locking protrusion provided in a groove located on a front surface of the main door that can selectively couple to the hook member. The hook member protrudes backward from a rear surface of the sub-door and can rotate within the sub-door about the pivot shaft in response to receipt of user input at the button.