Dynamic modeling and controller design are investigated for attitude maneuvers of liquid-filled spacecraft within a fully actuated system framework. The controller design is based on a dynamical model of liquid-filled spacecraft derived from the theorem of angular momentum, where the liquid sloshing is equivalent to a spherical pendulum with three degrees of freedom. The attitude system of the dynamical model is cast into a fully actuated system expressed in error quaternions. Then, an observer-based adaptive controller is proposed to achieve attitude maneuvers of liquid-filled spacecraft with external disturbances. The controller consists of a general fully actuated system control law, a nonlinear disturbance observer, and an adaptive system. The control law provides a basic structure of the controller. The nonlinear disturbance observer estimates and compensates for the unknown perturbation consisting of external disturbances and liquid sloshing. The adaptive system deals with the estimated errors of the unknown perturbation and enhances the stability of the system. The stability of the controlled system is proved by Lyapunov’s direct method. Numerical simulations demonstrate the effectiveness of the proposed controller.