The tension between selfishness and prosocial behavior is crucial to understanding many social interactions and conflicts. Currently little is known how to promote prosocial behaviors, especially in naturally occurring relationships outside the laboratory. We examined whether a psychological micro-intervention would promote prosocial behaviors in couples. Across two studies, we randomized dyads of couples to a micro-intervention (15 min), which increased prosocial behaviors by 28% and decreased selfish behaviors by 35% a week later in behavioral games in a dose-response manner. Using event sampling methodology, we further observed an increase in prosocial behaviors across one week that was most pronounced in participants who received the intervention. These results from the laboratory and everyday life are important for researchers interested in prosocial behavior and selfishness and have practical relevance for group interactions.