Described herein is a mounting system with measurement of the weight of the occupant for a seat of a motor vehicle; the mounting system has a bottom frame rigidly connected to the bodywork of the motor vehicle, a top frame, which supports the seat and is mounted in a floating way on the bottom frame, and a plurality of weight sensors, each of which is set between the bottom frame and the top frame and is connected to the bottom frame or to the top frame by means of a single bolt integrated in the weight sensor itself and locked by a nut; a surface of the nut facing the weight sensor has a spherical shape, and set between the nut and the frame is a washer having a spherical central surface, which reproduces in negative form the shape of the corresponding spherical surface of the nut.