Mercury (Hg) is an extremely toxic pollutant that threatens ecosystem balance and human health by persisting and accumulating in the environment and the food chain. High concentrations of Hg in plants result in the excessive accumulation of reactive oxygen species, which trigger oxidative stress, inhibit growth, and even cause death. Selenium (Se) is a trace element essential to human and animals. The application of Se in soil can inhibit Hg uptake by plants through the formation of Hg–Se insoluble complexes in the rhizosphere and/or roots. We propose that Se also can mitigate the toxicity of Hg by regulating the physiological mechanism that mediates the opposing effects of the plant antioxidant system and resulting peroxidation products. Earlier studies have investigated Se accumulation in plants by treating plant growth media or soil with selenate (Se[VI]). To the best of our knowledge, no study has determined the effect of exogenous Se(VI) on the physiological mechanisms underlying the changes in plant growth under Hg stress. Therefore, the main objective of this study was to investigate the effects of Se(VI) application on the detoxification of Hg via the physiological and biochemical responses of pak choi (Brassica chinensis).