A restraint system (10) for helping to protect an occupant (60) of a vehicle (20) having a roof (32) and a cabin (40) with a seat (50) for the occupant (60) includes an airbag (70) mounted to the vehicle roof (32). The airbag (70) has a stored condition and is inflatable to a deployed condition. The airbag (70), when deployed, extends from an upper end (90) connected to the vehicle roof (32) to a lower end (92) positioned adjacent the occupant (60). The airbag (70) has a U-shaped configuration including a first leg (104) and a second leg (106) pivotable relative to the first leg (104) in response to occupant (60) penetration into the second leg (106). A tether (120) has first and second ends (122, 124) each connected to the airbag (70) for limiting pivotal movement of the second leg (106) relative to the first leg (104).