The electronic and optical properties of binary semimetals GdSb and TbSb in the rocksalt cubic structure have been investigated. The spin-polarized band structure calculations were performed within GGA + U method accounting for strong electron correlations in rare-earth ions. The effect of spin-orbit interaction for the band structure is investigated. The resulting band features near the Fermi energy are found in a good agreement with the previous ARPES experimental data. Our first-principle calculations have shown that isostructural compounds GdSb and TbSb are electron-hole compensated semimetals with qualitatively similar electronic structures. A small pseudogap is observed in their densities of states. The energy dependencies of the real and imaginary parts of the complex dielectric permittivity of both materials are investigated by the spectroscopic ellipsometry in the range 0.078–5.6 eV. The experimental optical conductivity spectra are compared with the calculated ones. There is a reasonable agreement between experiment and theory on a number of spectral features formed by quantum transitions and related to the specifics of the electronic structures. The nature of the occurrence of intense absorption bands in the optical conductivity spectra of both compounds has been identified. The anomalous behavior of optical characteristics in the infrared region is associated with the semi-metallic properties of these materials.