Phase shift exceeding $\pi ∕2$ at $1\mathrm{THz}$ is demonstrated by using electrically controlled birefringence in a homeotropically aligned nematic liquid crystal (E7) cell, $570\mu \mathrm{m}$ in thickness. The driving voltage required for a phase shift of 90° is $125\mathrm{V}$ (rms). We demonstrate that the phase shifter works as an electrically switchable quarter-wave plate at $1\mathrm{THz}$ . The device can also be used as an electrically tuned phase compensator around the quarter-wave point near $1\mathrm{THz}$ .