Milk donation is allowed in Islam and considered a virtue, though according to Islam Sharia, feeding donated milk of other mothers leads to kinship between infants, complicating milk donation programs in Islamic countries. This study aimed to determine the knowledge and attitude of Iranian Muslim mothers regarding milk donation and milk banks. In this cross-sectional descriptive-analytic study, 634 mothers of infants below 1 year of age were recruited using cluster random sampling from health care centers in Tabriz, Iran. Data were collected by questionnaire. The findings revealed a low level of knowledge but relatively positive attitudes. Knowledge predictors were education level, income, type of birth, breastfeeding experience, encouragement to donate milk, and hearing about milk donation (p ≤ .02). Predictors of attitude were knowledge score and encouragement to donate milk (p ≤ .001). Comprehensive, culturally congruent education of mothers during pregnancy and post-pregnancy related to milk banks is recommended.