A motor vehicle seat including a seatback part and a sitting part, the sitting part including a base panel bordered at least partially by a side panel, and a mechanism mounting the seat on the vehicle floor panel in two different positions: a first accommodating position in which the side panel extends at least partially in an extension of the seatback part, between the seatback part and the sitting part base panel, and a second reversed position in which the base panel extends between the seatback part and the side panel of the sitting part. The motor vehicle seat further includes a mechanism locking the sitting part in at least one of the two accommodation and reversed positions, at least part of the locking mechanism belonging to the seatback part.