Current work presents a study about the effect of polyaniline (PANI) as an organic semiconductor on the photocatalytic performance of ZnSe nanoparticles (NPs). Pristine ZnSe NPs and ZnSe/PANI nanocomposites were synthesized by a simple and cost-effective co-precipitation method in the ambient conditions. X-ray diffraction (XRD), Raman, and Fourier-transform infrared spectroscopy (FTIR) results confirmed that, a heterojunction was created by ZnSe NPs and PANI composite. High-resolution transmission electron microscopy (HRTEM) image showed that the NPs were deposited by PANI completely and a core-shell structure was generated by ZnSe and PANI. Photoluminescence (PL) spectroscopy results showed the optical properties of the pristine ZnSe NPs and ZnSe/PANI nanocomposites were similar with lower defect emission intensity for the ZnSe/PANI nanocomposites. However, the PL and UV-Vis results revealed that, the PANI caused a decrease in band-gap value of the ZnSe/PANI nanocomposites in compared to the band-gap value of the pristine ZnSe NPs. In addition, X-ray photoelectron spectroscopy (XPS) results indicated that, the PANI caused a shift in the valance band (VB) and conduction band (CB) edges of ZnSe/PANI nanocomposites in compared with pristine ZnSe NPS. Finally, the photocatalytic performance of the products for removing of methylene blue (MB) and chromate ions was examined under a visible-light source irradiation. An enhancement photocatalytic performance for ZnSe/PANI nanocomposites in compared to the pristine ZnSe NPs was observed. Brunauer-Emmett-Teller (BET) results indicated that textural properties of the nanocomposites were decreased by PANI. According to the optical properties and band gap-value of the products a type-II heterojunction was created by band alignment of the ZnSe and PANI and such heterojunction was the most important factor for the enhancement photocatalytic performance of the ZnSe/PANI nanocomposites in compared to the photocatalytic activity of the pristine ZnSe NPs.