While the regulation of stress is usually thought of as an intrapersonal process, research suggests that relational factors such as attachment anxiety and avoidance play an influential role in stress regulation. Mindfulness, the process of intentionally paying attention to present moment experiences in a nonjudgmental way, has been associated with both enhanced romantic attachment security and stress regulation, though the precise role of attachment in mindfulness–stress paths remains unclear. The current study explores (1) the association between mindfulness and romantic partners’ physiological and subjective stress responses to a relationship conflict discussion and (2) the role of attachment anxiety and avoidance in statistically mediating that association. Heterosexual couples (n = 114 dyads) completed self-report measures of mindfulness and attachment approximately 1 week prior to a lab session involving a conflict discussion task. Participants rated state positive and negative affect and stress appraisals following the discussion, and five saliva samples were collected for cortisol assay to measure physiological stress. Results supported the proposed mediational model, with significant indirect effects of total mindfulness scores on stress outcomes through attachment. Specifically, mindfulness is related to lower cortisol levels during the conflict discussion via lower attachment avoidance and predicted less negative affect and more positive cognitive appraisals following the conflict discussion via lower attachment anxiety.