Here we report a planar polarization rotator using a nematic liquid crystal waveguide, which is subject to a gradient electric field in parallel to the waveguide substrate plane. The fabricated polydimethylsiloxane (PDMS) device has demonstrated the electrically-controlled polarization rotation at a switching time of 70 ms and a propagation loss of 9 dB/cm at 532 nm. The unique features such as planar form, electric control, and soft material allows integration into planar lightwave circuits and flexible photonics.