Abstract: The horizontal hinging pin (2) integral with the structure of the vehicle has a keeper (8) of annular shape having a profiled active surface (8a). The movable part (3) of the armrest comprises an operating assembly (22) mounted slidably on the frame of the armrest in the axial direction XX'. The operating assembly comprises a bolt (21) having an actuating surface comprising fingers (23) cooperating with the active surface (8a) of the keeper (8) in order to ensure locking of the armrest in a raised position. Unlocking of the armrest may be obtained by pushing a button (18) causing axial displacement of the operating assembly (22) and of the blocking finger (23) of the bolt (21). The armrest may be made to return into its lowered position by an elastic return member such as a spring.