A slip determination system for a vehicle, which is capable of improving the determination accuracy by avoiding erroneous determination of excessive slip of wheels when a state of the wheels, driven/braked by motors, is switched. In the slip determination system according to the present invention, when first and second motor rotational speeds NMOT1 and NMOT2, which are rotational speeds of rear motors which brake/drive rear wheels WRL and WRR, reach a reference rotational speed NMREF set based on wheel rotational speeds NWFL, NWFR, NWRL, and NWRR, it is determined that excessive slip has occurred in the rear wheels WRL and WRR. When the sign of a target torque TROBJ of the rear motors is inverted, the reference rotational speed NMREF is changed to a value more difficult to be reached by the first and second motor rotational speeds NMOT1 and NMOT2, or the excessive slip determination is inhibited.