Chemical pesticides and herbicides are one of the most important pollutants in urban, agricultural and industrial wastewaters. Improper discharge of these compounds into water bodies' cause harmful effects on both environment and human health. In this study, photocatalytic degradation of 2,4-Dichlorophenoxyacetic acid (usually called 2,4-D) was investigated using Mn-doped zinc oxide/graphene nanocomposite under light emitting diodes (LED) radiation. FTIR, AFM, DLS, Zeta potential, XRD, and SEM techniques were used to determine the characteristics of the nanocomposite. The effects of process-related parameters, such as the amount of nanocomposite, initial pH, 2,4-D concentrations, and contact time, on the photocatalytic degradation of the 2,4-D were studied. The results showed that the efficiency of photocatalytic degradation of 2,4-D decreased with an increase in the initial concentration of 2,4-D, while photocatalytic degradation efficiency increased by increasing the initial pH and the nano-catalyst content. The results showed that 66.2% of 2,4-D could be photocatalytically degraded using Mn-doped zinc oxide/graphene nanocomposite under LED radiation at optimal conditions (pH 5, initial Zn concentration of 10 mg L-1, nano-composite concentration of 2 g L-1, contact time of 120 min). Findings of this experimental study concluded that photocatalysis using Mn-doped zinc oxide/graphene nanocomposite under LED radiation could efficiently remove 2,4-D herbicide from aqueous media.