BACKGROUND: The World Health Organization (WHO) actively promotes breastfeeding as the optimal source of nourishment for infants and young children. However, not all newborns have access to breast milk, leading to deprivation of its nutritional benefits or incurring financial burdens from alternative feeding options. Establishing Human Milk Banks (HMBs) can help ensure equitable access to donated human milk. However, several factors may hinder breast milk donation. This study aims to identify the factors influencing milk donation to HMBs in Iran. METHODS: We conducted a case-control study involving mothers who had given birth at least one year prior to the study. The study included 51 cases (mothers who donated their milk to HMBs) and 153 controls. Data were collected using a questionnaire designed to gather retrospective information on individual health, social networks, and other relevant factors. Logistic regression analysis was performed to examine the relationships between breast milk donation and these factors. Additionally, qualitative data were collected through face-to-face interviews with HMB senior staff and mothers. Thematic analysis was employed to identify perspectives on factors influencing milk donation. RESULTS: Family factors, social influences, individual social welfare scores, and breast milk adequacy were significantly associated with increased milk donation. Among these, family support emerged as one of the strongest predictors of milk donation. CONCLUSION: To establish HMBs and promote human milk donation, health policymakers and planners should implement strategies that motivate mothers to donate. Evidence-based training and motivational programs for mothers and their families, which address barriers to milk donation, are essential to achieving this goal.