A seat-mounted airbag device includes an inflator that is operated when a vehicle collision is detected or predicted and ejects gas and an airbag body that includes a front-rear chamber and a tip chamber. The front-rear chamber is deployed forward of a seat through a clearance between a window and a head of an occupant from a side portion of a headrest on a window side by the gas ejected from the inflator and disposed between the window and the head of the occupant. The tip chamber is deployed inward in a seat width direction from an end portion of the front-rear chamber on a seat front side and disposed forward of a face of the occupant on the seat front side. The airbag body housed in the side portion includes an outward wound portion wound outward in a roll shape with a seat up-down direction as an axial direction.