The title compound was made by fusion of a stoichiometric mixture of the pure elements at 500 degreesC. The structure (monoclinic, P2(1)/c, Z = 8, a = 17.037(6), b = 12.253(1), c = 18.105(4) Angstrom, beta = 117.24(2)degrees) is made of alternating one- and two-dimensional sections with the same composition of Cs5In3As4, and can be considered as two coexisting polymorphic forms of the same compound. The chains and layers in these sections are made of repeating semicubane motifs of [In3As4](5-) that are connected to each other by In-As bonds in the chains, and by both In-As and In-In bonds in the layers. According to magnetic measurement and EHMO calculations, Cs5In3As4 is a closed-shell compound that fulfills the Zintl concept.