This article presents an application of the adaptive control method to semi-active suspension systems in the presence of unknown disturbance and parametric uncertainty. Due to the technical difficulties such as time delay and sensor noise, the road disturbance is assumed to be unmeasured. To overcome this problem, an observer is designed to estimate the disturbance. It is considered that the road profile consists of a finite number of the sum of sinusoidal signals with unknown amplitudes, phases and frequencies. After the parametrization of the observer, the adaptive control approach is employed to attenuate the effect of the road-induced vibrations using a magnetorheological damper. It is proved that the closed-loop system is stable, despite the adverse road conditions. Finally, the performance of the controller is illustrated with a hardware-in-the-loop simulation in which the system is subjected to sinusoidal and random profile road excitations. To demonstrate the benefits of the adaptive controller, the results are presented in comparison with a conventional proportional integral derivative (PID) controller.