This paper presents the design, fabrication, and characterization of a MEMS actuator with large static deflection as a waveguide-mounted variable millimeter-wave phase shifter. The actuator is composed of a pair of interdigitated microplates actuated by vertical comb-drives and suspended by SU-8 torsional springs. The SU-8 spring possesses a thin metallization top layer and a reverse-T-shaped cross-section enabling low torsional stiffness and high in-plane stability. A maximum mechanical deflection of 10.3° is obtained under a dc actuation voltage of 35 V. The dynamic characterization of the device shows that the resonance frequency of the torsional mode is well separated from the other three bending modes, confirming the designed low torsional stiffness and high in-plane stability. The torsional viscoelastic creeping is measured as a function of time at different loads and shows a maximum of 0.5° for an applied voltage of 27.5 V. A high operation cycle test is conducted and the metalized SU-8 spring withstands 800 million cycles without showing fatigue. RF measurements show that a variable mechanical deflection angle between 0° and 8.2° results in a variable transmission phase shift up to 58.0°. The measured insertion loss is always below 5.1 dB at 98 GHz, corresponding to a figure of merit of 11.5°/dB.