An electrically tunable achromatic polarization rotator has been developed based on the hybrid splay-twist (HST) liquid crystal. The proposed polarization rotator is advantageous over the conventional ones owing to the thin thickness (sub-100μm), continuous angular rotation, and achromatic operation across the entire visible spectrum. The tuning range of the polarization rotator is up to 90° via a simple electric field application; meanwhile, the degree of linear polarization (DOLP) remains. The rotation angle can be expanded to 180° by a tandem-cell geometry. The work will offer possibilities in the design of various optical systems and spatially polarization multiplexing elements.