Human milk donation is important for improving the development of preterm infants. However, despite the importance of donating the human milk, relatively little research has tested which factors predict this form of donation. This study assessed the association between the psychosocial factors and formalized milk donation to a nonprofit milk bank. This study used a cross-sectional design. Breastfeeding mothers (N = 556) completed measures assessing altruism, pride, instrumental and affective attitudes, subjective norm, perceived behavioral control, self-efficacy, anxiety, and intention to donate human milk to a nonprofit milk bank. We also assessed whether participants requested additional information about donating the human milk. Instrumental and affective attitude, subjective norm, and self-efficacy were positively associated with intention to donate milk. Self-efficacy and intention were also uniquely associated with requesting the additional information. The intention to engage in the formalized milk donation to a nonprofit milk bank appears to be more likely if women view this action as beneficial, believe significant others support the action and think they have the ability to undertake this action. Women who think they have the ability to undertake this action and are willing to donate are more likely to request additional information. These findings might inform future experimental research and campaigns on the human milk donation.