To provide a seat device capable of preventing a seat cushion in a seating posture from moving in a vehicle body width direction (seat width direction).

The seat device 15 includes: a fixed rail 35 which is supported by a vehicle body floor 11 for supporting a left seat cushion 41, and extends in the vehicle body width direction; a left slider 48 which is slidably connected to the fixed rail and supports a left movement mechanism 47; a left lock mechanism 131 for locking the left slider at predetermined positions P3, P4 of the fixed rail; and a left operation member 132 for unlocking the left lock mechanism. When the left seat cushion is in the seated posture out of the predetermined positions, a left regulation member 133 for regulating unlocking operation by the left operation member is provided.