The invention relates to a device (1) for dispensing a product (L), comprising: an element for connection to a container (R) containing the product; a piston (3) that is stationary in relation to the connection element; a cylinder body that moves around the piston, thereby defining a dosing chamber (100), the piston comprising a dosing inlet (35) for said chamber and the apex (64) of the dosing chamber comprising an outlet of the dosing chamber; and an inflow non-return valve (5) with a membrane for opening or closing the dosing inlet, the piston being in two parts, one of which forms a sealing joint with the cylinder body, the piston and the inflow non-return valve forming separate parts and being arranged such that the membrane is tightly clamped to the top of the piston.