A roof airbag for a vehicle. The roof airbag includes: a main cushion located at a roof panel of the vehicle and configured to be unfolded from a first side portion to a second side portion of the roof panel when being operated; a sub cushion coupled to a side surface of the extended main cushion and unfolded outside the main cushion; and an inflator located at the roof panel of the vehicle and connected to the main cushion or the sub cushion, and configured to inject gas into the main cushion and the sub cushion to unfold the main cushion and the sub cushion.