A functional analysis of prosociality considers how predispositions for prosocial behavior prompt, reinforce, and propagate kind behaviors in the real world. To examine the effects of practicing, receiving, and observing everyday prosociality—as well as the mechanisms underlying these effects—we randomly assigned employees in a Spanish corporate workplace (N = 111) to be Givers, Receivers, and Controls. Givers practiced 5 acts of kindness for a personalized list of Receivers over 4 weeks. We found that Givers and Receivers mutually benefited in well-being in both the short-term (e.g., on weekly measures of competence and autonomy) and the long-term (e.g., Receivers became happier after 2 months, and Givers became less depressed and more satisfied with their lives and jobs). In addition, Givers’ prosocial acts inspired others to act: Receivers paid their acts of kindness forward with 278% more prosocial behaviors than Controls. Our results reveal that practicing everyday prosociality is both emotionally reinforcing and contagious (inspiring kindness and generating hedonic rewards in others) and that receiving everyday prosociality is an unequivocally positive experience (which may further reinforce Givers’ actions). Prosociality’s benefits shed light on its surprising ubiquity in humanity compared with our closest evolutionary cousins.