In many spheres of life, from applying for a job to participating in an athletic contest to vying for a date, we face competition. Does the size of the competition pool affect our propensity to behave unethically in our pursuit of the prize? We propose that it does. Across four studies, we found that a larger (vs. smaller) number of competitors led participants to cheat more in a performance task to earn undeserved money. We also explored the psychological mechanisms of competition pool size to explain why and how being in a larger competition pool increases cheating. Our findings reveal a serial mediation pathway whereby having a larger number of competitors increases expectations of the absolute number of cheaters in the competition group, which heightens perceptions that cheating is an acceptable social norm, which leads to more cheating. We also examined and ruled out various alternative psychological mechanisms for this effect. We discuss the theoretical and practical implications of our finding that being in a large group of competitors can increase people’s propensity to cheat for personal gain.