A roof airbag for a vehicle is proposed. The roof airbag for a vehicle has an airbag cushion configured to be unfolded downward from an inner roof to protect a passenger seated on a seat, and the airbag cushion has a shape bent toward the passenger by a main tether and a sub-tether, thus securing a restraining force for the passenger and safely protecting the passenger from an impact, and the airbag cushion is unfolded downward while avoiding insertion between a roof-side portion of the roof and a headlining by the sub-tether. Specifically, the airbag cushion is prevented from being transversally contracted and an unfolding shape of the airbag cushion is maintained. Furthermore, the fluidity of a gas flow in the airbag cushion is secured, so that the airbag cushion is rapidly unfolded.