General and domain-specific (e.g., relationship-specific) mindfulness frequently predict salutary relationship outcomes. The present preregistered study examined whether general and relationship mindfulness predicted longitudinal change in positive and negative relationship quality via greater perceived partner responsiveness (PPR). One hundred couples completed a baseline lab session (Phase 1), a 14-day diary period (Phase 2), and a 2-month follow-up survey (Phase 3). Actor-Partner Interdependence Mediation Model analyses revealed that actors’ Phase 1 relationship mindfulness—but not general mindfulness—predicted increases in their own positive relationship quality from Phase 1–3 and decreases in their own negative relationship quality from Phase 1–3 indirectly via their own Phase 2 PPR. An exploratory alternate model testing if Phase 1 PPR predicted changes in Phase 1–3 relationship quality via Phase 2 relationship mindfulness did not reveal significant indirect effects. All results held when controlling for gender, age, and relationship length, and no partner effects emerged in any analysis. These findings further elucidate the relationship-enhancing role of mindfulness in couples and highlight PPR as a critical mediator explaining the link between domain-specific mindfulness and relationship quality. Implications for mindfulness-based training programs for couples are discussed.