An occupant protection system comprising: a wall portion of a vehicle cabin; a seat disposed such that a rear surface of a seatback in a seat front-rear direction faces the wall portion of the vehicle cabin; an airbag (32A; 32B) provided in the wall portion or the seatback and configured to support the seatback by inflating and deploying between the wall portion and the seatback; and an inflator (36; 36B) configured to generate gas to be supplied to the airbag (32A; 32B) in a case where a collision that causes an occupant seated in the seat to inertially move rearward in the seat front-rear direction is detected or predicted, wherein the airbag includes a seat deployment portion configured to support the seat including the seatback; characterized in that: the airbag (32A; 32B) has a vent hole through which the gas inside the airbag (32A; 32B) is discharged outside; and the airbag (32A; 32B) further includes: an upper deployment portion connected to an upper side of the seat deployment portion in a seat up-down direction and having the vent hole, the upper deployment portion being configured to inflate and deploy in an area above a head of the occupant in the seat up-down direction; a partition wall configured to separate the seat deployment portion from the upper deployment portion; and a check valve provided in the partition wall and configured to permit only a flow of the gas from the seat deployment portion to the upper deployment portion. a wall portion of a vehicle cabin; a seat disposed such that a rear surface of a seatback in a seat front-rear direction faces the wall portion of the vehicle cabin; an airbag (32A; 32B) provided in the wall portion or the seatback and configured to support the seatback by inflating and deploying between the wall portion and the seatback; and an inflator (36; 36B) configured to generate gas to be supplied to the airbag (32A; 32B) in a case where a collision that causes an occupant seated in the seat to inertially move rearward in the seat front-rear direction is detected or predicted, wherein the airbag includes a seat deployment portion configured to support the seat including the seatback; the airbag (32A; 32B) has a vent hole through which the gas inside the airbag (32A; 32B) is discharged outside; and the airbag (32A; 32B) further includes: an upper deployment portion connected to an upper side of the seat deployment portion in a seat up-down direction and having the vent hole, the upper deployment portion being configured to inflate and deploy in an area above a head of the occupant in the seat up-down direction; a partition wall configured to separate the seat deployment portion from the upper deployment portion; and a check valve provided in the partition wall and configured to permit only a flow of the gas from the seat deployment portion to the upper deployment portion. an upper deployment portion connected to an upper side of the seat deployment portion in a seat up-down direction and having the vent hole, the upper deployment portion being configured to inflate and deploy in an area above a head of the occupant in the seat up-down direction; a partition wall configured to separate the seat deployment portion from the upper deployment portion; and a check valve provided in the partition wall and configured to permit only a flow of the gas from the seat deployment portion to the upper deployment portion.