In an era of changing relationship norms, plans for marriage are an increasingly complex yet important indicator of the link between cohabitation and marriage. Despite qualitative evidence on this complexity, little is known about the nuances of marital plans and gender differences at the population level. This study introduces the concept of “informal” marital plans—cohabitations beginning with some intentions to marry that had yet to be formalized. Drawing on data of heterosexual cohabitors in their first coresidential union from the National Survey of Family Growth (2011–2015, n  = 5545), I examine the sociodemographic correlates of marital plans as well as their consequences for men’s and women’s union transitions. The results show significant gender differences in reports of marital plans at the time of moving in together, with women more likely to report engagement and men more likely to report informal marital plans. Although having any marital intentions is positively associated with transitioning to marriage for both genders, engagement is a significantly stronger predictor of marriage than informal marital plans. Pronounced gender differences are found with respect to the dissolution of first cohabitations, as both informal and formal marital plans are more protective against dissolution for men than for women. Distinguishing informal marital plans from engagement provides meaningful new insights into the role of cohabitation in modern American union formation.