Precocious exits from adolescence via early union formation are often argued to represent a strong risk factor for intimate partner violence (IPV) victimization. However, causal evidence for this claim is scant. This study examines the impact of teen union formation (before age 18) on experiences of IPV in Brazil and Colombia, where early family transitions are common and levels of interpersonal violence are high. Using data that allow instrumenting for teen union formation with age at menarche, results show that both Brazilian and Colombian women who start a co‐residential union before age 18 have a higher probability of experiencing psychological violence from partners. Early cohabitation is also linked to greater risk of past‐year sexual abuse among Black/Brown Brazilian women, and lifetime sexual IPV in Colombia, including among women who partnered once. Among testable potential pathways, age‐heterogamy (male partner being older) explains part of the results, but lower educational attainment among early cohabiting women emerges as a key driver in both countries. Education remains a powerful policy tool to confront both forms of gender‐based violence in South America.