People frequently engage in dishonest behavior at a cost to others, and it is therefore beneficial to study interventions promoting honest behavior. We implemented a novel intervention that gave participants a choice to promise to be truthful or not to promise. To measure cheating behavior, we developed a novel variant of the mind game—the dice‐box game—as well as a child‐friendly sender–receiver game. Across three studies with adolescents aged 10 to 14 years (N = 640) from schools in India, we found that promises systematically lowered cheating rates compared with no‐promise control conditions. Adolescents who sent truthful messages in the sender–receiver game cheated less in the dice‐box game and promises reduced cheating in both tasks (Study 1). Promises in the dice‐box game remained effective when negative externalities (Study 2) or incentives for competition (Study 3) were added. A joint analysis of data from all three studies revealed demographic variables that influenced cheating. Our findings confirm that promises have a strong, binding effect on behavior and can be an effective intervention to reduce cheating.