Bench seat for a motor vehicle, comprising: - two outer fixed rails (10, 20) each equipped with means (11, 21) for fixing to a chassis of the motor vehicle, - two outer movable rails (15, 25) which are mounted so as to slide on the outer fixed rails (10, 20) respectively, along a slide axis, and which are linked together by a crosspiece ensuring the paired sliding of the two outer movable rails (15, 25), and - a carrier structure which is coupled to the two outer movable rails (15, 25) and which delimits at least two separate seats, between which at least one anchoring means for a seatbelt is provided, characterized in that it comprises: - an intermediate fixed rail which is situated between the two outer fixed rails (10, 20) and which is equipped with means for fixing to the chassis of the motor vehicle, and - an intermediate movable rail which is mounted so as to slide on the intermediate fixed rail and which comprises: - a retaining means adapted to cooperate with said anchoring element in the case of accident to the motor vehicle, and - a connecting means cooperating with said crosspiece to ensure the linked sliding of the intermediate movable rail and the two outer movable rails (15, 25), this connecting means being open in a direction parallel to said slide axis in order to allow its assembly with said crosspiece by sliding of the outer movable rails (15, 25) on the outer fixed rails (10, 20). - two outer fixed rails (10, 20) each equipped with means (11, 21) for fixing to a chassis of the motor vehicle, - two outer movable rails (15, 25) which are mounted so as to slide on the outer fixed rails (10, 20) respectively, along a slide axis, and which are linked together by a crosspiece ensuring the paired sliding of the two outer movable rails (15, 25), and - a carrier structure which is coupled to the two outer movable rails (15, 25) and which delimits at least two separate seats, between which at least one anchoring means for a seatbelt is provided, - an intermediate fixed rail which is situated between the two outer fixed rails (10, 20) and which is equipped with means for fixing to the chassis of the motor vehicle, and - an intermediate movable rail which is mounted so as to slide on the intermediate fixed rail and which comprises: - a retaining means adapted to cooperate with said anchoring element in the case of accident to the motor vehicle, and - a connecting means cooperating with said crosspiece to ensure the linked sliding of the intermediate movable rail and the two outer movable rails (15, 25), this connecting means being open in a direction parallel to said slide axis in order to allow its assembly with said crosspiece by sliding of the outer movable rails (15, 25) on the outer fixed rails (10, 20). - a retaining means adapted to cooperate with said anchoring element in the case of accident to the motor vehicle, and - a connecting means cooperating with said crosspiece to ensure the linked sliding of the intermediate movable rail and the two outer movable rails (15, 25), this connecting means being open in a direction parallel to said slide axis in order to allow its assembly with said crosspiece by sliding of the outer movable rails (15, 25) on the outer fixed rails (10, 20).