Purpose: Glyphosate is a widely used non-selective herbicide. Previous studies have shown that glyphosate has genotoxicity, and that even low-doses of glyphosate can cause DNA damage. Melatonin is a hormone produced and secreted by the pineal gland that is known to be a potent anti-carcinogen, anti-oxidant, and genetic protector. This study was conducted to investigate the genoprotective effect of melatonin against glyphosate in human blood lymphocytes. Methods: Human peripheral blood was obtained from 15 young, healthy volunteers and cultured under four different toxicologic conditions. The four groups consisted of a control group, glyphosate only group (300 ng/mL), glyphosate with low level of melatonin group (50 μM), and glyphosate with high level of melatonin group (200 μM). The mean Sister Chromatid Exchange (SCE) frequency of each group was then analyzed. Results: Glyphosate exposed groups had a higher mean SCE frequency (10.33±2.50) than the control group (6.78 ±2.31, p<0.001). Interestingly, the group that received a low-level of melatonin had a lower mean SCE frequency (8.67±2.58) than the glyphosate-only group, while the group that received a high level of melatonin had a much lower mean SCE frequency (8.06±2.50) than the glyphosate-only group. There was statistical significance. Conclusion: Melatonin exerted a potent gene protective effect against the genotoxicity of glyphosate on human blood lymphocytes in a dose-dependent fashion.