A systematic approach in the design methodology, leading to the optimization of a differential reflection-type phase shifter (RTPS), is presented. The key parameters of the quadrature coupler and the reflective loads are analyzed for optimum RTPS performance, showing that: 1) equality between the coupling and through transmission coefficients provides the best return and insertion loss; 2) a minimized coupler reflection coefficient with negative phase optimizes the return loss and the phase performance; and 3) a wide tunable reflective load with minimized resistance assures lower insertion loss and larger phase range. The even–odd mode analysis leads to the coupler miniaturization. The differential microstrip line dimensions are determined by the minimum feature size of our fabrication process. By choosing the separation between the coupled lines, the even- and odd-mode characteristic impedances satisfy the derived design equations for equalized quadrature outputs and provide the matching capacitance. By exploiting the virtual ground of the differential structure, a novel RF MEMS reconfigurable short circuit is introduced for the reflective loads aiming at 180° phase shift range and low insertion loss. Over the 70–86-GHz range, the measurements show good agreement with the simulations. The measured RTPS achieves a 195.6° phase shift range with a phase error less than 8.6° and insertion loss error less than 0.7 dB. For all phase states, the reflection coefficients are below −18 dB, and at 74 GHz, the insertion loss is 4.4 ± 0.5 dB.