The photocatalytic properties of SnSe nanostructures (NSs) and SnSe/graphene nanocomposites with different graphene concentrations (5, 10, and 15 wt%/v) were investigated. The products were synthesized by a simple and cost-effective co-precipitation method. The samples obtained demonstrated that graphene concentration at an optimum amount was an important factor in enhancing the photocatalytic performance of the products. The graphene source was graphene oxide (GO) sheets and several characterization results indicated, which were used to remove Methylene blue (MB) dye, that the GO sheets were changed into reduced graphene oxide (rGO) sheets during the synthesis process. The optical properties of the products were studied using a room temperature photoluminescence (PL) spectrometer and it was observed that the near-band-edge (NBE) position of the samples was at the end of the red region between 729 and 756 nm of the electromagnetic spectrum, which was confirmed by a UV-vis spectrometer. The PL spectra of the samples also demonstrated three emissions from the violet, green, and orange regions of the visible spectrum, which were from different defects. The samples were annealed in a hydrogen and air atmosphere at 300 °C and it was found that defect concentrations were increased by annealing for the SnSe/rGO nanocomposites. The photocatalyst studies of the post-annealed samples revealed that the photocatalytic performance of the products was enhanced by annealing in hydrogen, while it was reduced by annealing in air. In addition to MB, the photocatalytic performance of the products for the degradation of phenol as a colorless pollutant was examined. It was observed that rGO in this process also had a significant role in the enhancement of photocatalytic performance. In fact, the electron spin resonance (ESR) test showed the role of rGO in photocatalytic activity very well.