A vehicle seat has a seatback movable between an upright position and a horizontal position, a seat bottom movable between a use position perpendicular to the seatback and a stowage position parallel to the seatback, and a lock mechanism for holding the seat bottom at the stowage position when the seat bottom is moved to the stowage position. When the seatback is moved to the horizontal position, the vehicle seat has a dive-down state where the seatback and the seat bottom are stacked on top of the other. The seat bottom has a detection lever that, when the seat bottom becomes the dive-down state, comes into contact with a floor panel and is displaced. In the lock mechanism, when the detection lever is displaced, the function of holding the seat bottom at the stowage position becomes ineffective.