A large literature links marriage to later life outcomes for children and adults. Marriage has declined markedly in the U.S. over the last 50 years, particularly among individuals with less than a baccalaureate degree, yet the causes of the decline are not well understood. In this paper we provide causal evidence on one potential mechanism for the observed marriage rate patterns: peer effects. We use administrative personnel data from the U.S. Army to study how peers influence marriage decisions for junior enlisted soldiers arriving to their first assignment from 2001-2018, a setting which features substantial variation in peer group marriage rates and conditional random assignment to peer groups. We find that exposure to the 75th versus 25th percentile of our identifying variation in peer marriage rates increases the likelihood that an unmarried individual marries within two years of assignment by 1.9 percent. We show that lateral peers and near supervisors alike influence marriage decisions and we argue that our results are most consistent with conformist behavior, where peers influence marriage decisions through role-modeling and group social norms. The effect of peers is larger for men, and for Black and Hispanic men, in particular. While the effect of peers attenuates after 36 months for white and Hispanic men, effects persist and continue to grow over time for Black men, suggesting that our results are not fully explained by re-timing. We benchmark our estimates against previous research and argue that the effect of peers on individual marriage decisions is economically meaningful.