<p>This study compared six dimensions of relationship quality among U.S. cohabiting adults in 1987 and 2013. Cohabitation has greatly changed over recent decades. Cohabitors nowadays are less likely to have plans to marry, more cohabiting relationships dissolve and fewer transition to marriage, the average duration is longer, and the number of serial cohabitors is growing. Cohabitation also increasingly may be functioning as an alternative to marriage. These trends signal that relationship quality may differ among more recent cohorts of cohabitors than among earlier cohorts, but it remains unclear whether the relationship quality of cohabiting unions has changed between cohorts. Data from the 1987–1988 National Survey of Families and Households (NSFH) and the 2013 Families and Relationships Survey (FRS) were used to conduct a cohort comparison of relationship quality among cohabitors. The analytic sample was composed of 1,638 cohabiting adults aged 19–65 in the United States. Multivariable ordinary least squares and logistic regression models investigated cohort differences in relationship quality. Relationship quality declined on three of six dimensions. Despite lower relationship quality on these measures, cohabitors in the more recent cohort were less likely than their earlier counterparts to report instability in their relationships. Relationship quality on the other two measures remained stable between cohorts. We also uncovered evidence that prior cohabitation may operate differently across cohorts. Future research should examine the implications of the changing landscape of cohabitation, especially for the health and well-being of cohabitors.</p>