The optimal motion planning of an underactuated rigid spacecraft using pseudospectral method is discussed. Firstly, the dynamic and kinematic equations of the spacecraft are derived, and for given initial and final attitudes, the performance to be optimized is selected as minimizing the rest-to-rest maneuver time. Moreover, the bounded control inputs are transformed to inequality constraints. Then, the optimal motion planning problem can be converted into a nonlinear programming problem by using Legendre pseudospectral method. The analytical and simulation results show that the proposed algorithm satisfies the constraints well and can be used on-line with its small computational amount and low computational complexity.