Exclusive breastfeeding is recommended for 6 months; however, many childbearing people wean their infants before 6 months. Psychosocial factors such as stress, social support and race are significant determinants of breastfeeding; however, few studies have longitudinally explored the effect of perceived stress and various forms of social support on exclusive breastfeeding. We used quantitative methodologies to examine exclusive breastfeeding, perceived stress and social support among 251 participants from the Postpartum Mothers Mobile Study. Participants between 18 and 44 years were recruited during pregnancy (irrespective of parity) and completed surveys in real-time via Ecological Momentary Assessment up to 12 months postpartum from December 2017 to August 2021. We measured perceived stress with the adapted Perceived Stress Scale and perceived social support with the Multi-dimensional Social Support Scale. Received social support was measured using a single question on breastfeeding support. We conducted a mixed-effects logistic regression to determine the effect of stress, race and social support on exclusive breastfeeding over 6 months. We examined the moderation effect of perceived social support and breastfeeding support in the relationship between perceived stress and exclusive breastfeeding. Black, compared with White, participants were less likely to breastfeed exclusively for 6 months. Participants who reported higher perceived stress were less likely to breastfeed exclusively for 6 months. Perceived social support moderated the relationship between perceived stress and exclusive breastfeeding (odds ratio: 0.01, 95% confidence interval: 0.001-0.072). However, breastfeeding support directly increased the likelihood of exclusive breastfeeding over 6 months. Perceived stress is negatively associated with exclusive breastfeeding. Birthing people who intend to breastfeed may benefit from perinatal support programs that include components to buffer stress.