A seat for a vehicle, that includes a headrest with a headstall for supporting the head of a passenger in the seat, wherein the occupant may or may not wear a protection helmet depending on the circumstances. The headstall can assume at least three stable positions: a first position for the cervical support of the head of the seat passenger in which the headrest provides a cervical support function with or without wearing a helmet; a second position for the occipital support of the head of the seat passenger in which the headrest provides an occipital support function without wearing a helmet; a third position for the occipital support of the head of the seat passenger in which the headrest provides an occipital support function while wearing a helmet.