Building upon the evidence supporting the interpersonal benefits of trait mindfulness, efforts to understand mindfulness in the context of intimate relationships (i.e., relationship mindfulness) have been made to better predict relationship outcomes. To further understand the relationship benefits of mindfulness, it is crucial to take a closer look at for whom and under what conditions trait mindfulness translates into relationship mindfulness. In this study, we examined the moderating effects of one’s own and their partner’s mental well-being on the positive link between trait mindfulness and relationship mindfulness. Specifically, we expected this link to be weaker for those who themselves or their partners reported worse mental well-being as poor mental well-being may pull couples inward, away from showing up mindfully in relationships. A sample of 115 different-sex married couples completed an online questionnaire that included measures of trait mindfulness, relationship mindfulness, and markers of mental well-being (i.e., depression, anxiety, loneliness). A series of actor-partner interdependence moderation models showed that, consistent with our hypotheses, the link between husbands’ trait mindfulness and relationship mindfulness was weaker if husbands or their wives reported higher depression or anxiety. Therefore, couples may benefit more if mindfulness-based couple interventions directly target how partners can use mindfulness skills to stay connected even when facing mental health challenges. This study also further highlights the importance of considering various relational contexts for mindfulness research and interventions.