A thermostatic device including a thermostatic element and a stopper, axially movable about a stationary seat of a housing, opening and closing a passage circulating fluid and connected to a movable portion of the thermostatic element. During expansion of a thermoexpansible material of the thermostatic element, the movable portion drives the stopper axially relative to the seat. The stopper includes a seal, resting tightly against the seal when the fluid circulation passage is closed by the stopper, a first part, with an inner bore receiving the movable portion, and a second part, separate from the first part, the seal being axially charged between the first and second parts. The first and second parts are made of metal, the second part is mounted around the first part, the part facing the axis of the second part forms a peripheral edge attaching said second part to the outer surface of the first part, and the first part forms an axial abutment for the edge of the second part, positioning the first and second parts axially relative to one another by applying a predetermined clamping load to the seal.