Barium containing quaternary chalcogenides have recently attracted significant interest due to their promising thermoelectric applications. The novel semiconductor quaternary selenide BaSmCuSe3 of the orthorhombic space group Cmcm with the KZrCuS3 structure type was obtained for the first time using a reductive selenidation method. Structural parameters and optical properties of BaSmCuSe3 were discussed in comparison with the isostructural sulfide BaSmCuS3. Replacing sulfur with selenium in a quaternary chalcogenide leads to a narrowing of the band gap from 2.30 eV to 1.93 eV. The crystal structure and optical properties of the synthesized herein compound were discussed within the ab initio approach. The reults of calculations are in good agreement with the experimental characteristics.