This paper presents an adaptive sliding-mode (ASM) control methodology for a vehicle steer-by-wire (SbW) system. First, the SbW system is modeled as a second-order system from the steering motor input voltage to the front-wheel steering angle. For simplicity, the self-aligning torque and friction arising from the tire-to-ground contact are regarded as external disturbance acting on the SbW system. Next, an ASM controller is designed for the SbW system, which can not only cope with the parametric uncertainties in the plant model but also estimate the coefficient of the self-aligning torque effectively. The stability of the ASM control system is proved in the sense of Lyapunov and the guidelines for selecting the control parameters are given. Finally, experiments are carried out for steering control to respectively follow a slalom path and a circular path under various road conditions. It is shown that the proposed ASM controller can achieve stronger robustness against various road conditions leading to significantly smaller tracking errors in comparison with a conventional sliding-mode controller and a linear H ∞ controller.