The invention concerns a foldable seat (1) for a rollator or a wheelchair, said seat comprising: - a first rigid plate (2) and a second rigid plate (3), said plates being adjacently connected along a longitudinal axis (XX') so as to be rotatably moveable around said longitudinal axis (XX') between an unfolded position, in which their plane surfaces (2a, 3a) are approximately aligned, and a folded position, in which their plane surfaces (2a, 3a) define an angle less than 180°, and preferably, an angle approximately equal to zero, said plates (2, 3) being also configured so as to be pivotally connectable to a frame of a rollator or a wheelchair, and - a hinge (10) pivotally connecting said first plate (2) to said second plate (3), wherein said hinge (10) comprises at least a first cylinder (11) integral with said first plate (2), said first plate (2) projecting over said first cylinder (11), at least a second cylinder (12) integral with said second plate (3), said second plate (3) projecting over said second cylinder (12), and a pivot pin (14) extending through axial through-bores and/or apertures (11c, 12c) of said first and second cylinders (11, 12), said pin (14) defining said longitudinal axis (XX'), characterized in that said seat (1) further comprises a locking/unlocking system (20) configured to be actuated by hand by the user so as to lock, respectively unlock, said first and second plates (2, 3) into, respectively from, said unfolded position and/or said folded position, said locking/unlocking system (20) being at least partially lodged inside said first and/or said second cylinder (11, 12).
