A vehicle steering control system includes a variable transmission ratio device that rotates an output shaft relative to an input shaft so as to change the transmission ratio, and a locking device that is switched between a lock-on state in which the transmission ratio between the input shaft and the output shaft is inhibited from being changed and a lock-off state in which the transmission ratio between the input shaft and the output shaft is allowed to be changed. The variable transmission ratio device includes an electric motor and a speed reduction mechanism. The locking device inhibits the input shaft and a rotor of the electric motor from rotating relative to each other when placed in the lock-on state, and allows the input shaft and the rotor to rotate relative to each other when placed in the lock-off state.