The invention provides a vehicle steering system. A variable transmission ratio mechanism (30) is provided on an intermediate portion of a steering shaft (2), and is able to change a transmission ratio between a shaft (4a) on the steering wheel (1) side and a shaft (4b) on the steered wheel (8) side by transmitting the sum of rotation of the first shaft (4a) and rotation of a rotary shaft of a motor to the second shaft (4b). The variable transmission ratio mechanism (30) is provided with a lock mechanism that restricts the operation of the variable transmission ratio mechanism (30). When the fact that a rate of change in torque that acts on the steering shaft (2) exceeds a predetermined rate is detected, a period of time until the torque that acts on the steering shaft (2) reaches a torque determination value is estimated. On the condition that the estimated time is shorter than or equal to a predetermined period of time, the lock mechanism is activated.