The invention refers to a
convertible car seat (1) that can take on various configurations such as seat, shelf, document case and flat, comprising: a seat (2) and a backrest (3); a support made up of two frames (6), the sides of which are made up of rigid elements (8, 9, 10, 11, 8′, 9′, 10′, 11′) connected to each other by means of joints (A12, A13, A14, A15) placed on each of their ends on an axle (12, 13, 14, 15) in such a way that the support can be folded in the manner of a foldable
quadrilateral, in which at least one of the joints (A12, A13, A14, A15) is lockable, the seat (2) is mounted lockably connected to the third axle (14), arms (16) are placed on either side of the seat, the backrest (3) is mounted on one end (18) of the aforementioned arms (16) by means of at least one lockable joint (19). The invention also relates to a vehicle that includes at least one such seat and a method for transforming such a seat.