A crash box for a vehicle may include a front connection portion that is connected to both ends of a back beam of the vehicle, a rear connection portion that is connected to a side member of the vehicle, an intermediate connection portion for connecting the front connection portion and the rear connection portion, which is formed as a hollow box shape and through which a cut cross-section of a heptangular shape is formed in a vehicle width direction, and a plurality of reinforcement ribs formed at an inner space of the intermediate connection portion.