A new solid solution has been prepared in the system LiNb3O8-CuNb3O8, and the impacts of chemical composition and crystal structure have been investigated for the resulting band gap sizes and photocatalytic activities for water reduction to hydrogen under visible light. All members of the solid solution were synthesized by solid-state methods within evacuated fused-silica vessels, and their phase purities were confirmed via powder X-ray diffraction techniques (space group P2(1)/a, a = 15.264(5)-15.367(1) angstrom, b = 5.031(3)-5.070(1) angstrom, c = 7.456(1)-7.536(8) angstrom, and beta = 107.35(1)-107.14(8)degrees, for 0 <= x <= 1). Rietveld refinements were carried out for the x = 0.09, 0.50, and 0.70 members of the solid solution, which reveal the prevailing isostructurality of the continuous solid solution. The structure consists of chains of (Li/Cu)O-6 and NbO6 octahedra. The optical band gap size across the solid solution exhibits a significant red-shift from similar to 3.89 eV (direct) to similar to 1.45 eV and similar to 1.27 eV (direct and indirect) with increasing Cu(I) content, consistent with the change in sample color from white to dark brown to black. Electronic structure calculations based on density-functional theory methods reveal the rapid formation of a new Cu 3d(10)-based valence band that emerges higher in energy than the O 2p band. While the pure LiNb3O8 is a highly active UV-photocatalyst for water reduction, the Li1-xCuxNb3O8 solid is shown to be photocatalytically active under visible-light irradiation for water reduction to hydrogen.