Photocatalysis is considered to be one of the possible routes to solve energy crises at a low cost. Graphitic-C 3 N 4 combined with other semiconductors at the nanoscale has demonstrated a promising potential in photocatalysis application owing to the heterojunction configuration. In this study, Ag/g-C 3 N 4 /CuNb 2 O 6 -4 (mass ratio of CuNb 2 O 6 to g-C 3 N 4  = 1:4) composites were prepared by photo-deposition of Ag particles on the hydrothermally synthesized g-C 3 N 4 /CuNb 2 O 6 -4 composites. The microstructure, morphology, and light absorption property of the photocatalysts were characterized by X-ray diffractometry, scanning electron microscopy, transmission electron microscopy, X-ray photoelectron spectroscopy, and UV–Vis diffuse reflectance spectroscopy. The photocatalytic properties of photocatalysts and active species responsible for the degradation of RhB dye under visible light were investigated. After loading Ag particles on the g-C 3 N 4 /CuNb 2 O 6 -4 composites, the resulting Ag/g-C 3 N 4 /CuNb 2 O 6 -4 composites show a bandgap of ~ 2.31 eV and a significant increase in photocatalytic efficiency. In particular, the photocatalytic activity of 11% Ag/g-C 3 N 4 /CuNb 2 O 6 -4 composite (Ag mass fraction = 11%) is nearly seven times that of orthorhombic CuNb 2 O 6 and 3.8 times that of g-C 3 N 4 /CNO-4 composite. The Z-scheme heterojunction has been successfully formed in the Ag/g-C 3 N 4 /CuNb 2 O 6 -4 composites and it enhances the photocatalytic performance significantly with the best degradation efficiency of 88% after 2 h.