Using a randomized wait-list controlled design, this study evaluated the effects of a novel intervention, mindfulness-based relationship enhancement, designed to enrich the relationships of relatively happy, nondistressed couples. Results suggested the intervention was efficacious in (a) favorably impacting couples' levels of relationship satisfaction, autonomy, relatedness, closeness, acceptance of one another, and relationship distress; (b) beneficially affecting individuals' optimism, spirituality, relaxation, and psychological distress; and (c) maintaining benefits at 3-month follow-up. Those who practiced mindfulness more had better outcomes, and within-person analyses of diary measures showed greater mindfulness practice on a given day was associated on several consecutive days with improved levels of relationship happiness, relationship stress, stress coping efficacy, and overall stress.