Insufficient milk supply is the most common reason for premature breastfeeding cessation. Breast hypoplasia is one reason why women may be inherently unable to make a full milk supply. This review aimed to systematically explore the relationship between breast hypoplasia and breastfeeding duration, milk supply, and lactation onset. Medline, CINAHL, ProQuest Central, and the Cochrane Library databases were searched using the keywords “insufficient glandular tissue” or “mammary hypoplasia” or “breast hypoplasia” or “mammary gland hypoplasia” or “droopy breasts” or “snoopy deformity” or “tubular breast*” or “tuberous breast*” AND breastfeeding or “breast feeding” or breast-feeding or lactation. After initially screening 20 records, including reference lists, 9 full texts were assessed for eligibility; 2 were excluded as no breastfeeding outcomes were reported, leaving 7 studies ( = 42 women). The studies in this review drew on results from the oldest included study and plastic surgery literature to define breast hypoplasia. Most women in this review (40/42) ceased exclusive breastfeeding before 1 month postpartum. One case study reported 24-hour milk production, which was 52 mL at 26 weeks postpartum. The relationship between breast hypoplasia and breastfeeding outcomes is underresearched. The co-occurring medical conditions (e.g., polycystic ovary syndrome) of some women provide avenues for future research into the possible pathogenesis of breast hypoplasia resulting in insufficient milk supply. Research is needed to evaluate the reliability of measuring and classifying markers of breast hypoplasia, and prospective studies can help determine the role of breast hypoplasia in milk production. PROSPERO registration number CRD42020191228.