Postpartum systemic oxidative stress is common in dairy cows and can lead to health complications and production loss. While the gut microbiota can influence host health and stress, its role in dairy cow postpartum oxidative stress was unclear until a recent study found a strong link between the fecal microbiota and postpartum oxidative stress in dairy cows. Compared to low oxidative stress (LOS) cows, those with high oxidative stress (HOS) had altered bacterial community composition, including 16 species from 9 genera that were negatively correlated with oxidative stress. There were also changes in the microbiota’s functional behavior and related metabolites, and the findings suggested that changes in glutathione synthesis played a causative role in oxidative stress. Specifically, HOS cows had reduced microbial metabolism of amino acids involved in glutathione synthesis. Compared to mice with microbiota transplanted from LOS cows, mice with a microbiota from HOS cows had elevated oxidative stress, as well as reduced plasma glutathione peroxidase and glutathione. While more research is needed, the findings suggest that the gut microbiota contributes to oxidative stress by reducing glutathione production and could lead to novel strategies to alleviate oxidative stress in dairy cows.