Oxysulfoselenide semiconductor photocatalysts absorb light at longer wavelengths than the corresponding oxysulfides. However, the synthesis of oxysulfoselenides is challenging due to excessive particle growth and the limited availability of metal selenide precursors. In this study, a La5Ti2CuS5O7 (LTCSO) oxysulfide was annealed with Se powder in sealed, evacuated quartz tubes to obtain LTCSO:Se photocatalysts, and the properties of these materials were investigated. Se was found to be incorporated into the LTCSO upon heating at 973 K or higher, and the Se/(S + Se) ratio was increased to a maximum of 0.3 upon repeating the heat treatment twice. The addition of Se extended the absorption edge of the LTCSO and thus increased its photocatalytic H-2 evolution activity at longer wavelength. Even so, the apparent quantum yield at shorter wavelengths was reduced, which is similar to the results obtained for La5Ti2Cu(S1-xSe)(5)O-7(LTCS1-xSexO) solid solutions. Overall water splitting was achieved by constructing photocatalyst sheets using LTCSO:Se and LTCS1-xSexO as hydrogen evolution photocatalysts and BiVO4 as an oxygen evolution photocatalyst. Heat treatment with Se is evidently an effective method for the transformation of oxysulfide photocatalysts to oxysulfoselenides that promote photocatalytic H-2 evolution and have longer absorption edge wavelengths.