This paper considers an adaptive terminal sliding mode control for systems with unknown control direction and non-vanishing disturbance. As opposed to the previous treatments of systems with unknown control direction and exogenous disturbance, where the control gain is designed with large magnitude to compensate for the worst case scenarios, this work presents a control structure that adjusts the input gain to the minimal value to maintain the sliding motion. The presented control structure enables both the sliding surface and the state to reach the equilibrium in finite time. Simulation results are provided to show the efficacy of the developed control structure.