Objectives The association of mindfulness and romantic relationship outcomes such as partnership quality and satisfaction is well-established; however, the mechanisms of action are not yet clear. The current study tested conflict resolution styles and closeness as possible mediating factors. We hypothesized that trait mindfulness would increase the use of constructive conflict resolution styles (positive problem solving), decrease the use of destructive styles (conflict engagement, withdrawal, and compliance), and promote feelings of closeness between partners, which in turn would predict positive relationship outcomes (namely partnership quality, partnership satisfaction, and sexual satisfaction). Methods A total of 209 individuals (86% German, 76% female, mean age = 32 years) living in a relationship (31% married) participated in an online questionnaire. Results Mediation analyses revealed that positive problem solving mediated the association between mindfulness and partnership quality with b = .09 (95% CI = .03-.17), mindfulness and partnership satisfaction with b = .07 (95% CI = .02-.13), and mindfulness and sexual satisfaction with b = .04 (95% CI = .00-.10). Furthermore, a mediating role of withdrawal and closeness was shown for individual relationship outcomes. Conclusions Findings suggest that more positive problem solving, less withdrawal, and more closeness are mechanisms by which mindfulness is associated with positive relationship outcomes. The results of our study thus broaden our understanding of the processes that underlie fulfilling romantic relationships and, in turn, underline the positive effects of mindfulness.