Hexagonal semiconductors such as 4H SiC have important high-frequency, high-power, and high-temperature applications. They require accurate knowledge of ordinary and extraordinary relative permittivities, ε and ε, perpendicular and parallel, respectively, to the c axis of these semiconductors. However, due to challenges for suitable test setups and precision high-frequency measurements, little reliable data exists for these semiconductors especially at millimeter-wave frequencies. This work includes both ε and ε of 4H SiC from 57 to 330 GHz, as well as their temperature and humidity dependence. For example, at room temperature, real ε and ε are constant at 9.77 ± 0.01 and 10.20 ± 0.05, respectively. By contrast, the ordinary loss tangent increases linearly with the frequency f in the form of (4.9 ± 0.1)  10−16 f and less than 1  10−4 over most millimeter-wave frequencies. Finally, both ε and ε have weak temperature coefficients on the order of 10−4 /°C.