We used latent growth curve modeling to examine the effectiveness of a relationship education intervention (Relationship U, or RU) on rates of extradyadic involvement in a sample of 380 college students in committed romantic relationships. RU is designed to be integrated into existing college courses; it educates students about partner selection, making healthy relationship transitions, communication skills, and the potentially negative consequences of cheating in romantic relationships and how to prevent its occurrence. Participants who received the intervention reported trajectories of less extradyadic involvement over time relative to control participants. Being female was not associated with less extradyadic involvement at baseline, but it did predict less extradyadic involvement over time across both intervention and control conditions. Implications for dissemination of relationship education are discussed.