Here we present for the very first time a single-crystal investigation of the Cu-poor Zn-rich derivative of Cu2ZnSnS4. Nowadays, this composition is considered as the one that delivers the best photovoltaic performances in the specific domain of Cu2ZnSnS4-based thin-film solar cells. The existence of this nonstoichiometric phase is definitely demonstrated here in an explicit and unequivocal manner on the basis of powder and single-crystal X-ray diffraction analyses coupled with electron microprobe analyses. Crystals are tetragonal, space group I (4) over bar4, Z = 2, with a = 5.43440(15) angstrom and c = 10.8382(6) angstrom for Cu2ZnSnS4 and a = 5.43006(5) angstrom and c = 10.8222(2) angstrom for Cu1.71Zn1.18Sn0.99S4.