Eight new quaternary selenides CsSmZnSe3, CsTbZnSe3, CsDyZnSe3, CsHoZnSe3, CsErZnSe3, CsTmZnSe3, CsYbZnSe3, and CsYZnSe3 have been synthesized with the use of high-temperature solid-state experimental methods. These compounds are isostructural with KZrCuS3, crystallizing with four formula units in the orthorhombic space group Cmcm. The structure of these CsLnZnSe3 compounds is composed of ∞2[LnZnSe3-] layers separated by Cs atoms. The Ln atom is octahedrally coordinated by six Se atoms, the Zn atom is tetrahedrally coordinated by four Se atoms, and the Cs atom is coordinated by a bicapped trigonal prism of eight Se atoms. Because there are no Se-Se bonds in the structure, the oxidation state of Cs is 1+, that of Ln is 3+, and that of Zn is 2+. CsYbZnSe3 exhibits an antiferromagnetic transition at 11 K, whereas CsSmZnSe3 does not follow a Curie-Weiss law. The remaining rare-earth compounds are paramagnetic, and the calculated effective magnetic moments of the rare-earth ions agree well with their theoretical values. Optical absorption data on face-indexed single crystals of CsSmZnSe3, CsErZnSe3, CsYbZnSe3, and CsYZnSe3 demonstrate that the optical band gap changes by more than 0.75 eV with the composition and by as much as 0.20 eV with the crystal orientation. The optical band gaps range from 2.63 eV (CsSmZnSe3, CsErZnSe3) to 1.93 eV (CsYbZnSe3) for the (010) crystal face and 2.56 eV (CsErZnSe3) to 1.88 eV (CsYbZnSe3) for the (001) crystal face. The difference in the optical band gap of the (010) face vs the (001) face varies from +0.05 eV (CsYbZnSe3) to +0.20 eV (CsSmZnSe3).