Previous research has demonstrated a strong link between prosocial behavior – particularly autonomous prosocial behavior – and well-being. Little is known, however, about whether and how autonomy might be boosted in the context of everyday kindnesses. We tested the effect of supporting students’ autonomy on well-being gains from practicing acts of kindness in a six-week randomized experimental study in the United States and South Korea. As predicted, performing kind acts while receiving autonomy support led to greater improvements in well-being than performing kind acts without autonomy support or engaging in comparison activities (i.e. focusing on one’s academic work, with or without autonomy support). Notably, these well-being improvements were mediated by feelings of autonomy, competence, and relatedness. The current study is one of the first to demonstrate the causal effect of autonomous prosocial behavior on well-being, as well as the psychological mechanism (i.e. need satisfaction) explaining this effect.