Zinc stannate (Zn2SnO4) powder was prepared by a hydrothermal process at 200 °C for 12 h. The material was characterized by X-ray-diffraction (XRD) and N2 adsorption–desorption isotherms. The synthesized sample presented a pure phase, an average nanocrystal size of about 19 nm, a surface area (BET) of about 41.8 m2 g−1 and total pore volume of about 0.19 cm3 g−1. Its photocatalytic activity was tested by the degradation of the leather azo-dye, Direct Black 38, in aqueous solution under UV irradiation. Adsorption kinetic data showed that the pseudo-second-order model was the most appropriate for the dye studied. Adsorption onto the Zn2SnO4 surface followed the Langmuir isotherm. The catalyst showed highly photocatalytic activity towards degradation of the dye, almost equal to that of the TiO2-P25 Degussa photocatalyst. The results indicate that Zn2SnO4 could be employed for the removal of dyes from wastewaters.