In this paper, an islanded DC microgrid is controlled using nonlinear adaptive decentralized controllers. The decentralized controllers are designed for the DC microgrid components which supply power (e.g., renewable energy sources and other standby generators) and store energy such as battery energy storage systems (BESSs). The power sources and BESSs in DC microgrids are decoupled using feedback linearization technique and adaptive controllers are then designed by considering the DC-link capacitor of these components as unknown. The adaptation law is used to estimate the value of the DC-link capacitor while ensuring the desired value of the common DC-bus voltage. The effectiveness of the proposed controller is evaluated on a DC microgrid with a fuel cell, solar photovoltaic (PV) unit, BESS, and DC loads. Simulation results are carried out under different operating conditions and compared with an existing proportional integral (PI) controller.