Objective To examine whether prosocial behaviors help sustain a positive mood, we tested the dynamic reciprocal associations between prosocial behavior and positive affect (PA) in daily life. A second aim was to examine whether the personality traits Neuroticism and Extraversion moderate these associations. Method The study included a community sample (N = 553). Participants completed an electronic diary assessing prosocial behavior and PA three times a day over 30 days. A subsample of 322 participants filled out the NEO Five‐Factor Inventory to assess Neuroticism and Extraversion. Multilevel autoregressive models were performed to examine the within‐person bidirectional associations between prosocial behavior and PA and possible moderation by Neuroticism and Extraversion. Results Within individuals, more PA was followed by more prosocial behavior at the next assessment, and more prosocial behavior was followed by more PA. The effect of prosocial behavior on PA was stronger for individuals high on Neuroticism. Extraversion did not moderate the associations under study. Conclusions The findings indicate that prosocial behavior and PA reinforce each other in daily life. Prosocial behavior seems most beneficial for individuals high on Neuroticism.