Bus voltage control is a crucial issue in dc microgrids. Constant power loads (CPLs) exhibit negative incremental impedance characteristics and tend to affect the operation of the microgrid system. To this end, most of the existing methods concentrate on ensuring the steady-state performance of the system while the transient performance is often overlooked. Hence, this article proposes an adaptive voltage-guaranteed controller for the dc/dc buck converter feeding CPLs to enhance both steady-state and transient performances. First, a constrained function is utilized to incorporate the voltage constraint into the modeling of a buck converter. Second, the adaptive update laws are constructed to estimate uncertainties/disturbances for different operation conditions. Finally, the design guideline of the proposed adaptive controller is presented using the backstepping theory. Through the aforementioned design procedures, the proposed controller can keep the bus voltage working within the prespecified range even when large-signal disturbances happen. The stability of the closed-loop system is proved by using the Lyapunov stability theory. Both simulations and experimental studies are conducted to verify the efficacy of the proposed controller.