Agricultural pesticides are known as the most important causes of water pollution due to their high environmental stability and production of carcinogenic products. Chlorpyrifos is broadly utilized to control pests in agricultural, residential, and commercial applications. The release of chlorpyrifos into water and wastewater sources may affect human health because of its chronic toxicity to aquatic organisms. Therefore, its degradation seems necessary. The present study aimed to investigate the photocatalytic efficiency of biochar nanocomposite based on grapefruit skin modified with Fe3O4 and CdS nanoparticles (biochar/CdS-Fe3O4) in the degradation of chlorpyrifos pesticide as an agricultural contaminant. The synthesized nanocomposites were characterized using field emission scanning electron microscopy (FESEM) with energy dispersive X-Ray spectroscopy (EDS), X-ray diffraction (XRD), and transmission electron microscopy (TEM). The synthesized nanocomposite showed excellent photocatalytic performance due to the improved separation of electrons and holes produced by CdS and the inhibition of agglomeration of these nanostructures by combining them at the biochar surface. According to the results, 97% of chlorpyrifos was degraded by the prepared nanocomposite during the photocatalytic process. The photocatalytic activity of the synthesized nanocomposite was dependent on the parameters of time, pH, pesticide concentration, amount of used photocatalyst, which were optimized. The pseudo-first-order kinetic model derived from the Langmuir-Hinshelwood equation well describes the behavior of the experimental data in this study. It was also reused several times after washing to evaluate the stability of the synthesized photocatalyst. This study introduced a new type of photocatalyst for pesticide degradation.