A vehicle motion control system for a vehicle having a single front wheel, a right wheel and a left wheel. The control system includes (a) a rollover-probability judging portion configured to judge whether or not a vehicle-body acceleration falls in a high rollover-probability region; and (b) a rollover-prevention control executing portion configured, when the acceleration falls in the high rollover-probability region, to execute a rollover prevention control for controlling motion of the vehicle so as to reduce the probability of rollover of the vehicle. The rollover-probability judging portion is configured to obtain a direction and a magnitude of the acceleration by composing a component of the acceleration in a longitudinal direction of the vehicle and a component of the vehicle-body acceleration in a width direction of the vehicle. The high rollover-probability region is defined by a threshold whose amount varies depending on the direction of the vehicle-body acceleration.