The novel oxysulfides La2Ta2ZrS2O8 (LTZSO), La2Ta2TiS2O8 (LTTSO), and La2Nb2TiS2O8 (LNTSO) were synthesized, and their crystal structures, electronic structures, and photocatalytic activities for water splitting under visible light were investigated. Density functional theory calculations showed that these compounds are direct-band-gap semiconductors. Close to the conduction band minimum, the main contribution to the band structure comes from the d orbitals of Zr or Ti ions, while the region near the valence band maximum is associated with the 3p orbitals of S ions. The absorption-edge wavelength was determined to be 540 nm for LTZSO and 700 nm for LTTSO and LNTSO. An analysis of the crystal structure using synchrotron X-ray diffraction revealed that these compounds contained antisite defects at transition metal ion sites, and these were considered to be the origin of the broad absorption at wavelengths longer than that corresponding to band-gap excitation. LTZSO was revealed to be active in the oxygen evolution reaction from aqueous solution containing a sacrificial electron acceptor under visible-light illumination. This result was supported by the band alignment and flat-band potential determined by photoelectron spectroscopy and Mott-Schottky plots.