A vehicle steering system in which an operating member that is operated for steering is not mechanically coupled to a steering mechanism and the steering mechanism is driven by a steered system motor comprising: a rotation angle sensor used to detect a rotation angle (θ SM) of a rotor of the steered system motor; and a target rotor angle computation unit that computes a target rotor angle (θ SM *) that is a target value of the rotation angle (θ SM) of the rotor of the steered system motor based on a steering state, characterized in that the steered system motor is formed of a brushless motor, the vehicle steering system further comprising: a rotor angle estimator that estimates the rotation angle (θ SM) of the rotor of the steered system motor based on an induced voltage of the steered system motor; a signal superimposing unit used to superimpose a rotation maintaining command signal for continuously rotating the steered system motor on the target rotor angle (θ SM *) computed by the target rotor angle computation unit; a malfunction detector that detects occurrence of a malfunction of the rotation angle sensor; and a selector, wherein when a malfunction of the rotation angle sensor is detected by the malfunction detector, the selector changes a control mode from a first control mode, in which the steered system motor is controlled based on the target rotor angle computed by the target rotor angle computation unit or the target rotor angle on which the rotation maintaining command signal has been superimposed by the signal superimposing unit and a value detected by the rotation angle sensor, to a second control mode, in which the steered system motor is controlled based on the target rotor angle on which the rotation maintaining command signal has been superimposed by the signal superimposing unit and the rotation angle of the rotor, which is estimated by the rotor angle estimator. a rotation angle sensor used to detect a rotation angle (θ SM) of a rotor of the steered system motor; and a target rotor angle computation unit that computes a target rotor angle (θ SM *) that is a target value of the rotation angle (θ SM) of the rotor of the steered system motor based on a steering state, characterized in that the steered system motor is formed of a brushless motor, the vehicle steering system further comprising: a rotor angle estimator that estimates the rotation angle (θ SM) of the rotor of the steered system motor based on an induced voltage of the steered system motor; a signal superimposing unit used to superimpose a rotation maintaining command signal for continuously rotating the steered system motor on the target rotor angle (θ SM *) computed by the target rotor angle computation unit; a malfunction detector that detects occurrence of a malfunction of the rotation angle sensor; and a selector, wherein when a malfunction of the rotation angle sensor is detected by the malfunction detector, the selector changes a control mode from a first control mode, in which the steered system motor is controlled based on the target rotor angle computed by the target rotor angle computation unit or the target rotor angle on which the rotation maintaining command signal has been superimposed by the signal superimposing unit and a value detected by the rotation angle sensor, to a second control mode, in which the steered system motor is controlled based on the target rotor angle on which the rotation maintaining command signal has been superimposed by the signal superimposing unit and the rotation angle of the rotor, which is estimated by the rotor angle estimator. a rotor angle estimator that estimates the rotation angle (θ SM) of the rotor of the steered system motor based on an induced voltage of the steered system motor; a signal superimposing unit used to superimpose a rotation maintaining command signal for continuously rotating the steered system motor on the target rotor angle (θ SM *) computed by the target rotor angle computation unit; a malfunction detector that detects occurrence of a malfunction of the rotation angle sensor; and a selector, wherein when a malfunction of the rotation angle sensor is detected by the malfunction detector, the selector changes a control mode from a first control mode, in which the steered system motor is controlled based on the target rotor angle computed by the target rotor angle computation unit or the target rotor angle on which the rotation maintaining command signal has been superimposed by the signal superimposing unit and a value detected by the rotation angle sensor, to a second control mode, in which the steered system motor is controlled based on the target rotor angle on which the rotation maintaining command signal has been superimposed by the signal superimposing unit and the rotation angle of the rotor, which is estimated by the rotor angle estimator.