A restraint system is provided for helping to protect an occupant of a vehicle having a roof and side structure defining a cabin with a seat for the occupant. The restraint system includes an airbag having a stored condition within the vehicle seat and being inflatable to deploy upwards in the cabin to a deployed condition between the occupant and the side structure. The airbag extends from an upper end positioned adjacent the roof to a lower end positioned adjacent a base of the seat. The airbag extends in a forward-rearward direction of the vehicle sufficient to span the entire depth of the seat in the forward-rearward direction.