Research has established that competing head to head against a rival boosts motivation and performance. The present research investigated whether rivalry can affect performance over time and in contests without rivals. We examined the long-term effects of rivalry through archival analyses of postseason performance in multiple high-stakes sports contexts: National Collegiate Athletic Association (NCAA) Division I Men's Basketball and the major U.S. professional sports leagues: National Basketball Association (NBA), National Football League (NFL), Major League Baseball (MLB), and National Hockey League (NHL). Econometric analyses revealed that postseason performance of a focal team’s rival in year N predicted that focal team's postseason performance in year N+ 1. Follow-up analyses suggested that the performance boost was especially pronounced when one’s rival won the previous tournament. These results establish that rivalry has a long shadow: A rival team’s success exerts such a powerful motivational force that it drives performance outside of direct competition with one's rival and even after a significant delay.