<p>Stem lettuce is a nutritious vegetable rich in dietary fiber, vitamins, and bioactive compounds, but fresh-cut processing accelerates browning, oxidative stress, and quality deterioration during storage. This study investigated the effects of ethanol vapor on browning inhibition, antioxidant capacity, and reactive oxygen species (ROS) metabolism in fresh-cut stem lettuce. Samples were treated with ethanol vapor at 0, 0.5, 1.0, and 2.0 mL/L and stored for 12 days. Among the treatments, 0.5 mL/L ethanol vapor most effectively delayed browning by suppressing the activities of peroxidase, polyphenol oxidase, and phenylalanine ammonia-lyase, while reducing the accumulation of superoxide anion radicals and hydrogen peroxide. This treatment also enhanced antioxidant defense systems by increasing the activities of superoxide dismutase, catalase, ascorbate peroxidase, and glutathione reductase, as well as non-enzymatic antioxidant capacity, as indicated by elevated FRAP, DPPH, and ABTS scavenging activities. By the end of storage, FRAP and DPPH values in the 0.5 mL/L group were approximately 1.8 and 2-fold higher than those of the control. Overall, ethanol vapor at 0.5 mL/L effectively alleviated oxidative stress, maintained ROS homeostasis, and delayed enzymatic browning, demonstrating its potential as a practical preservation strategy to improve postharvest quality and extend the shelf life of fresh-cut stem lettuce during cold storage.</p>