A modular firearm grip assembly has a frame defined by a front grip strap and a rear grip strap. The front grip strap defines a front lateral attachment point and rear lateral attachment point, and the rear grip strap has a first attachment coupling interface. A grip extension defined by a front extension strap and a rear extension strap is coupled to the frame at the respective one of the front lateral attachment point and the rear lateral attachment point thereof. The grip extension includes a second attachment coupling interface engaged to the first attachment coupling interface of the frame.