A rear seat side airbag device includes: a case provided inside a vehicle cabin side portion in correspondence with a rear seat, the case being open substantially toward a vehicle front side; an inflator accommodated inside the case, the inflator configured to generate gas at a time of vehicle collision; and a side airbag accommodated inside the case and is configured to receive a supply of gas from the inflator and inflates and expands, the side air bag including a distal end portion being fixed to a region of the case or a vehicle body, which region is at a vehicle width direction inner side with respect to the inflator.