Here, we report a metasurface for polarization converters in the terahertz (THz) spectral range. The THz metasurface consists of a metal mesh with subwavelength isotropic apertures and a nematic liquid crystal (LC) that is a uniaxially anisotropic dielectric medium. The LC is embedded and aligned homogeneously in the apertures. We calculated the complex transmittance and complex reflectance of the LC-embedded metal mesh (LC metasurface) for the eigen-polarization states using the finite-difference time-domain method, and we then investigated the effective electromagnetic parameters. The LC metasurface exhibits birefringence of approximately 1, which is one order of magnitude larger than that of the LC, in the transmission band. We also investigated the relationship between the geometrical parameters of the LC metasurface and transmissive retardation. These results demonstrate that the LC metasurface is useful for transmissive polarization converters in the THz spectral range.