A restraint system for helping to protect an occupant of a vehicle having a floor and a cabin with a floor for the occupant includes an airbag. The airbag has a stored condition on the vehicle floor and is inflatable to a deployed condition aligned with the occupant in a forward-rearward direction of the vehicle. The airbag is configured to utilize the vehicle floor as a reaction surface for restraining the movement of the airbag in response to occupant penetration into the airbag.