In a longitudinal study of a large sample of Americans, the authors found that people with a low socioeconomic status (SES) gained more weight during the Covid-19 pandemic, further exacerbating their vulnerability to the SARS-CoV-2 virus. The association between SES and weight gain was mediated by stress, but not by the other environmental or psychological factors suggested by prior research (e.g., temporal focus). A serial mediation model demonstrated that stress both decreased energy expenditures (through reduced physical activity) and increased energy intake (through higher and less healthy food intake). A follow-up study revealed that the early effects of the pandemic on weight and behavioral changes persisted 20 months later. Furthermore, stress levels decreased among people with a higher SES, but remained high for those with a lower SES. These findings demonstrate how the COVID-19 pandemic exacerbated health inequalities and provides insights for market-based and government solutions.