Adjustable seat device for a vehicle, comprising: -a base portion (10), -a seat portion (20) that is movable with respect to the base portion (10) between a first position, referred to as retracted, and a second position, referred to as extended, -an articulation system (30) forming a deformable parallelogram, which is connected to the base portion (10) and to the seat portion (20) and comprises at least one diagonal support arm (31) connected diagonally to the deformable parallelogram, characterized in that the diagonal support arm (31) is designed to have a first length when the seat portion (20) is in the first position and a second length when the seat portion (20) is in the second position.