Self-reported marijuana use has been associated with poor relationship functioning and decreased stability over time. The present study examined the behavioral interactions of couples with concordant and discordant patterns of marijuana use during conflict, using individual self-reports and observation by independent coders. Heavy drinking community couples (N = 149) participated in a conflict resolution paradigm. Interactions were recorded and coded by naïve coders. Approximately 30% of the sample reported past year marijuana use. Actor-Partner Interdependence Models and analysis of covariance (ANCOVA) were used to evaluate the individual and interactive effects of dyadic marijuana use on maladaptive relationship functioning. A Robust Actor × Partner Marijuana Use interaction was detected for a range of behavioral outcomes, assessed by both self-report and direct observation, including relationship satisfaction, anger experience, patterns of demand and withdrawal during conflict, constructive behaviors, and overall relationship quality. Specifically, couples in which both partners used or abstained from marijuana displayed more adaptive relationship functioning across indicators relative to couples in which only 1 partner identified as a marijuana user. This pattern was particularly strong for couples in which the female partner used marijuana and the male partner did not. Couples with discordant, rather than concordant, marijuana use displayed distinct conflict resolution behaviors that were consistent with the long-term negative relationship outcomes that have been observed in previous studies. (PsycINFO Database Record