Generalized ellipsometry can uncover the optical properties of anisotropic materials, in which the light–matter interaction alters the polarization state. In the terahertz frequency range, generalized ellipsometry has been infrequently realized due to the challenge of rapidly controlling and measuring THz polarization. Here, we report the development and calibration of a high-efficiency terahertz time-domain generalized ellipsometer based on two dual-channel photoconductive antennas. The fiber-c