We present a polymeric optical phase retarder that is electrically tunable by a dielectric elastomer actuator. The soft material device affords a large tuning range ( $14\pi$ at $\lambda =488\mathrm{nm}$ ) combined with high accuracy in optical path length and low drift rate $(8.3\mathrm{nm}∕\min )$ . Furthermore, the phase retarder is not sensitive to polarization, introduces a wavefront distortion $<\lambda ∕30$ , and tolerates high power densities $(>141\mathrm{kW}∕{\mathrm{cm}}^2)$ . We show the dynamics for periodic phase modulation and demonstrate a simple drive technique for fast phase stepping. The polymer-based device is inexpensive, easy to fabricate, and its design can be adapted to specific applications.