In this work, a magnetorheological (MR) damper valve is designed with the primary objective of controlling swing-phase damping in an above-knee prosthesis. Initially, a swing phase model of the desired single axis knee incorporating MR damper was modelled. The control parameters that govern damping force and displacement of the damper were identified and optimized to enable the prosthetic knee to produce near normal swing phase trajectory for ground walking as obtained from experimental data. Then, the MR damper valve is optimally designed by selecting typical performance indices of the damper for the intended application. A multi-objective optimization problem is formulated where the MR damper valve is constrained in a desired cylindrical volume defined by its radius and height. Effects of the geometrical design variables of the valve are analytically investigated by mapping finite element analysis (FEA) numerical responses with response surface method (RSM). The results show that the MR damper with designed damper valve enables the prosthetic knee to achieve near to normal swing phase trajectory, and compare to the existed MR damper, up to 71 % reduction by weight has been achieved. © 2018, The Korean Society of Mechanical Engineers and Springer-Verlag GmbH Germany, part of Springer Nature.