The device (1) comprises: a body (2) in which is formed a receptacle (3) having a mouth (4) and intended to receive a quantity of washing agent, a cover (5) connected to the body (2) and movable between a position in which the mouth (4) of the said receptacle (3) is closed and a position in which the mouth is open, and a return spring tending to move the cover (5) towards the open position. The cover (5) has a first portion (5a) hinged to the body (2) about a first axis (7) parallel to the mouth (4) of the receptacle (3) and translatable along guides (6) provided in the body (2), and a second portion (5b) connected to a crank element (8) by hinging about a second axis (9) parallel to the first axis (7). The crank element (8) is, in turn, hinged to the body (2) about a third, fixed axis (10) which is parallel to the first and second axes (7, 9).