A side airbag device includes: an inflator; a cushion main body that is disposed at a side of a seat of a vehicle and that is expandable from the side toward a front of the vehicle by a gas; a cushion protrusion that is disposed at a vehicle front side with respect to the cushion main body and that is expandable to project further toward the front of the vehicle by receiving the gas from the cushion main body; a tether that is string-shaped, the tether bridging from the cushion protrusion over a predetermined position of the side of the seat, the tether maintaining the cushion protrusion in a state in which the cushion protrusion is folded back toward a rear of the vehicle; a sensor that receives information from the vehicle; and a tether cutter that cuts the tether according to the information obtained by the sensor.