NaLa1-xBixS2 solid solutions with tunable bandgaps were synthesized, and their optoelectronic structures and photocatalytic performance were investigated via experimental and theoretical approaches. The solid-solution powders with various La/Bi ratios were synthesized with Na2CO3, La2O3 and Bi2O3 as precursors and via sulfurization with flowing CS2 at 800 °C for 2 h. The Vegard’s law behavior of cell parame-ters showed a perfect Bi/La solid solution in the cubic NaLa1-xBixS2 with the associated linear variation of the lattice constants. On the con-trary, the combination of diffuse reflectance UV−Vis spectroscopy with density functional theory (DFT) calculations employing the HSE06 functional reveals a monotonic but non-linear variation of the bandgap of the solid solution. While consistent valence band maxi-mum (VBM) was obtained in NaLa1-xBixS2—consisting mainly of S 3p orbitals—the conduction band minimum (CBM) was contributed by discrete Bi orbitals present at more positive potential than La. As a result, the slight inclusion of Bi caused a drastic shift in the bandgap, and 24% Bi substitution provided an absorption edge closer to that of pure NaBiS2. Systematic DFT calculations on NaLa1-xBixS2 deter-mined the optoelectronic properties for improved photovoltaic and photocatalytic performance with a Bi-rich sample rather than a La-rich counterpart; i.e., there were larger absorption coefficients, smaller effective masses, and larger dielectric constants for Bi-rich samples versus La-rich samples. The NaLa1-xBixS2 particles decorated with Pt nanoparticles show maximum hydrogen evolution performance with x = 0.02-0.06 of Bi samples consistent with the compensating effects between photon absorption capacity and loss of electromotive force with decreasing bandgap.