A roof mounted airbag is proposed. The roof mounted airbag includes: an airbag cushion having an upper portion fixed to a roof of a vehicle and configured to be expanded downward when gas is supplied therein; a vent hole formed on the airbag cushion, and configured to discharge the gas in the airbag cushion to the outside of the airbag cushion during opening of the vent hole; and a closing tether having a first end fixed to the roof of the vehicle, and a second end connected to the vent hole to close the vent hole when tension is applied thereto.