This paper presents a robust control approach for semi-active suspensions with electrorheological (ER) dampers considering system parameter uncertainties and control input constraint. By representing the suspension with parameter uncertainties in a polytopic form and applying a norm-bounded approach to handle control input constraint, the design of this controller is derived as solving a finite number of linear matrix inequalities (LMIs) with optimised H_¿ performance on ride comfort. Numerical simulations on a quarter-car suspension with an ER damper are performed to validate the effectiveness of the proposed approach. The obtained results show that the designed controller can achieve good suspension performance in spite of the variations on sprung mass and ER damper time constant with limited control input.