The Zn3M2O8 (M = Nb, Ta) compounds with layered crystal structures were prepared by a conventional solid-state reaction method, and their electronic-band structures, optical properties, and photocatalytic activities were investigated. The prepared compounds were characterized by X-ray diffraction (XRD), scanning electron microscopy (SEM), UV–Vis diffuse reflectance, and Mott–Schottky measurement. The UV–Vis diffuse reflectance spectra revealed Zn3Nb2O8 and Zn3Ta2O8 exhibited band gaps of 4.1 and 4.5 eV, respectively. From the electronic-band-structure calculations using DFT method, it was found that the valence band in both compounds was constructed by the hybridization of Zn 3d and O 2p orbitals, whereas the conduction band was consisted of Nb 4d (Zn3Nb2O8) and Ta 5d (Zn3Ta2O8) orbitals. The photocatalytic activity of both compounds was examined by the measurement of the H2 production rate in an aqueous methanol solution under UV light irradiation. The Zn3Ta2O8 with a larger band gap exhibited higher activity than Zn3Nb2O8, which was attributed to its higher reduction potential resulting from the difference of its electronic-band configuration.