A fixed-time adaptive control scheme is proposed for Buck DC/DC converters. Firstly, the fixed-time sliding mode surface and corresponding controller are developed for the system with known parameters to guarantee that the output voltage error converges to the equilibrium point within a fixed time, and the upper bound of the convergence time is independent of system initial states but determined by the control parameters. In the case that the value of some parameters including resistances and inductances is not precisely obtained under the effect of temperature and external disturbances, the adaptive update laws are designed for estimating all the unknown parameters of the Buck converters. Then, the fixed-time adaptive controller is proposed such that the output voltage error can converge to a small region around the equilibrium point within a fixed time. Simulation and experimental comparison results show the effectiveness of the proposed control scheme.