In this paper, a new adaptive viscous damper (AVD) is proposed and required equations are developed to describe its mechanical behavior. As opposed to conventional adaptive devices, the proposed damper is capable of adapting its own mechanical properties without any need for other devices such as sensors, processing unit, actuators, energy supplies, and wired or wireless connections. Eliminating such equipment not only reduces costs, but also removes related time lag and improves the efficiency of the control system. The proposed AVD includes a cylinder filled with viscous fluid and a piston with a nozzle at its head. The passing area of the nozzle is variable and as a result, the device can cover a range of damping coefficients. For non-extreme excitations, the damping coefficient is relatively small and consequently the damping force is reduced. Conversely, when extreme movements occur, the nozzle contracts a bit and generates relatively large control forces in order to protect the main structure efficiently. The mechanical model of the AVD is created in OpenSees and the damper is implemented in a two-story building example subjected to different earthquake records. The results show that compared to a typical viscous device, the proposed AVD can reduce the mean values of displacement, acceleration, and base shear by up to 52.5, 62.9, and 44.4% and increase the energy dissipation by up to 94.3% for extreme cases. Moreover, for non-extreme cases, the AVD shows a more flexible behavior and reduces the unfavorable damping forces.