Hybrid composites of Zn–In mixed metal oxides (ZnIn-MMO) and g-C₃N₄ were synthesized by a facile thermal decomposition of Zn–In layered double hydroxide (ZnIn-LDH) and melamine mixture precursors. The structural and optical properties of the ZnIn-MMO/g-C₃N₄ composites were characterized by powder X-ray diffraction, transmission electron microscopy, UV-vis diffuse reflectance spectroscopy, X-ray photoelectron spectra, photoluminescence spectra, electron spin resonance and transient absorption spectra. The results indicated that ZnIn-MMO nanoparticles were well distributed over the surface of the g-C₃N₄ sheets formed in situ. Compared with pristine ZnIn-MMO, the as-synthesized ZnIn-MMO/g-C₃N₄ nanohybrids showed stronger absorption in the visible light region. Furthermore, the ZnIn-MMO/g-C₃N₄ composite with a g-C₃N₄ amount of 36 wt% exhibited significantly enhanced photodegradation activity for Rhodamine B under visible light irradiation, in comparison with pure g-C₃N₄ and ZnIn-MMO, which was attributable to the unique heterostructure of the ternary semiconductor coupling system composed of g-C₃N₄, In₂O₃ and ZnO in the composites, facilitating efficient transportation and separation of the photogenerated electron–hole pairs and thus the continuous generation of reactive oxygen species. The present finding provides a simple approach for fabricating new types of visible-light-induced g-C₃N₄-based semiconductor composite photocatalysts for pollutant degradation in advanced oxidation processes.