Background: Breast engorgement is a physiological condition that appears in the first 14 days after childbirth and is characterized by interstitial edema, vascular and lymphatic congestion, and painful breast enlargement resulting from a sudden surge in milk volume. Aim of the study: Was to evaluate the effect of supportive educational intervention on knowledge and self-care practices regarding the prevention of breast engorgement among primiparous women. Subjects and Methods: Research design: Quasi-experimental design (pre- & post-test) one group was used to investigate the current study. Setting: The study was conducted at the outpatient antenatal clinic at Zagazig University Hospital. Subjects: A purposive sample of 70 primiparous women was included in the study. Tools of data collection: Three tools were used: a structured interview questionnaire, a questionnaire of knowledge of primiparous women regarding breastfeeding, breast engorgement, and an observational checklist for primiparous women's self-care practices regarding the prevention of breast engorgement. Results: After the implementation of the supportive educational intervention, there was a highly statistically significant increase in primiparous women's knowledge and self-care practices regarding the prevention of breast engorgement, and the vast majority of them didn't have breast engorgement during the first postpartum week. In addition, there was a highly significant statistical positive correlation between total primiparous women's knowledge score and their total self-care practices score regarding the prevention of breast engorgement pre- and post-intervention. There was a highly significant statistical negative correlation between total primiparous women's knowledge score, total self-care practices score, and their complaint of breast engorgement post-intervention. Conclusion: The research hypothesis was achieved as the majority of the primiparous women had good knowledge and satisfactory levels of self-care practices regarding the prevention of breast engorgement after the implementation of the supportive educational intervention. Recommendations: Maternity nurses and other healthcare professionals should teach women how to prevent breast engorgement as a primary topic in antenatal sessions. Additionally, it helps to increase their knowledge and self-care practices by distributing pamphlets, booklets, and drawings of the strategies of preventing breast engorgement during antenatal visits.