In this paper, slewing maneuver of a flexible spacecraft with large angle of rotation is considered and assuming structural frequency uncertainties a robust minimum-time optimal control law is developed. Considering typical bang-bang control commands with multiple symmetrical switches, a parameter optimization procedure is introduced to find the control forces/torques. The constrained minimization problem is augmented with the robustness constraints, which in turn increases the number of switches in the bang-bang control input to match the total number of the constraint equations. The steps of the solution algorithm to obtain the time optimal control input are discussed next. The developed control law is applied on a given satellite during a slewing maneuver, and the simulation results show that the robust control input with just few switching times can significantly lessen the vibrating motion of the flexible appendage in the presence of structural frequency uncertainties, which reveals the merits of the developed control law.