The new zero-dimensional selenide Ba2AsGaSe5 was synthesized via a solid-state reaction at 900 °C. It belongs to the orthorhombic space group Pnma with a = 12.632(3) Å, b = 8.9726(18) Å, c = 9.2029(18) Å, and Z = 4. In the structure, the As atom adopts trigonal-pyramidal coordination owing to the stereochemically active 4s(2) lone pair electrons and the Ga atom is tetrahedrally coordinated with four Se atoms. The AsSe3 trigonal pyramids share edges with GaSe4 tetrahedra to form novel [AsGaSe5](4-) clusters, which are further separated from each other by Ba(2+) cations. The optical band gap was determined as 1.39 eV according to UV-vis-NIR diffuse reflectance spectroscopy. Interestingly, the photocatalytic behavior investigated by decomposing rhodamine B indicates that the compound displays a 6.5 times higher photocatalytic activity than does P25.