This paper proposes a nonlinear multivariable robust output feedback controller for a continuous crystallizer with fines trap which is modeled by a highly uncertain population balance model. Initially, the method of moments is applied to the population balance model to reduce it to a small set of differential equations. which is subsequently employed for controller synthesis by using a combination of geometric control methods with Lyapunov techniques. The performance and robustness of the proposed controller are successfully tested through simulations and are shown to be superior to the ones of a nonlinear controller which does not account For uncertainty.