Aim: This study among moderately fit male athletes investigated changes in oxidative stress and antioxidant levels after supplementation with Eri silkworm cornflakes. Method: Male university athletes (n = 18, age 19–21 years) were enrolled in a randomized crossover trial. They participated in either the supplementation period (consuming 0.20 g protein/kg body weight Eri silkworm daily) or the control period (no supplementation). Each period lasted 4 weeks, with a washout period of 4 weeks in between. Results: Compared to the control period, reduced glutathione concentration (p = 0.019) and reduced/oxidized glutathione ratio (p < 0.001) were significantly higher, and oxidized glutathione (p = 0.018) and malondialdehyde (p = 0.002) concentrations were significantly lower in the supplementation period. Conclusion: Eri silkworm supplementation as cornflakes for 4 weeks improved oxidative stress and antioxidant status in male university athletes. This supplement may be an alternative to support optimal antioxidant status for athletic individuals. Thai Clinical Trials Registry (TCTR): TCTR20180303003.