Pro-social behaviors have been associated with enhanced well-being, but what psychological mechanisms explain this connection? Some theories suggest that beneficence-the sense of being able to give-inherently improves well-being, whereas evidence from self-determination theory (Weinstein & Ryan, 2010) shows that increases in well-being are mediated by satisfaction of innate psychological needs for autonomy, competence, and relatedness. Here we simultaneously assess these two explanations. Study 1 (N=335) used a cross-sectional survey with an Internet sample to develop a measure to assess beneficence satisfaction. The next two cross-sectional Internet-sample studies tested mediators between pro-social behavior and general well-being (Study 2, N=332) and situational peak moment well-being (Study 3, N=180). A fourth study (N=85) used a diary method with university students to assess daily fluctuations in well-being associated with needs and beneficence. It was shown across all studies that both the three psychological needs and beneficence satisfaction mediate the relations between pro-social actions and well-being, with all four factors emerging as independent predictors. Together, these studies underscore the role of autonomy, competence, and relatedness in explaining the well-being benefits of benevolence, and they also point to the independent role of beneficence as a source of human wellness.