In this work, the NiO and ZnO combined with g-C3N4 to form ternary heterojunction nanocomposite (NiO/ZnO/ g-C3N4) photocatalyst through facile co-precipitation method and evaluated for its photocatalytic degradation activity against azo dyes upon excitation with solar light. NiO/ZnO is a p-n type transition metal oxide semiconductor nanocomposite which proved to be producing photocatalytic degradation of industrial dyes. Here, the influence of g-C3N4 on physicochemical properties and photocatalytic degradation of azo dyes by NiO/ZnO (NZ) nanocomposite was analyzed. Physicochemical properties of the nanocomposite NiO/ZnO/g-C3N4 (NZC) was analyzed with characterization techniques XRD, UV-visible, FTIR, RAMAN, PL, SEM with EDX, HR-TEM, XPS, and EIS. Under solar light excitation, the potential of NiO/ZnO/g-C3N4 was analyzed in the degradation of azo dyes (MB and CR). The photodegradation results showed that the NiO/ZnO/g-C3N4 ternary nanocomposite possesses enhanced photocatalytic properties than the pure elements, and its binary nanocomposites (NiO/ZnO (NZ), ZnO/g-C3N4 (ZC) and NiO/g-C3N4 (NC)). NiO/ZnO/g-C3N4 nanocomposite degraded 99 % and 98 % of the methylene blue and congo red dyes, respectively. This enhanced photocatalytic degradation of dyes might be due to internal charge transfer within the heterojunctions. Also, presence of graphitic carbon nitride had elevated the quantity of reactive oxygen species, increased the solar light captivation, and decreased the rate of recombination of photo-excited charge carriers. Scavenger test and stability studies were carried out to evaluate the photocatalyst's performance. The obtained results confirm that NiO/ZnO/g-C3N4 can be employed in the visible photocatalytic degradation of industrial dyes.