Abstract: The seat structure comprises a seat proper and a backrest. The seat proper is mounted on the floor of the vehicle to pivot about a first transverse axis and the backrest is mounted on the seat proper to pivot about a second transverse axis. At least one link is connected at one end to the base part of the backrest to pivot about a third transverse axis and connected at the opposite end thereof to the floor of the vehicle to pivot about a fourth transverse axis. The seat proper and the link form two crossed arms of a device for deploying in the horizontal position the backrest relative to the seat proper upon a pivoting of the latter through about 180.degree. about the first axis in the forward direction relative to the vehicle.