Objective: This study investigates whether the adult attachment styles of support partners in a cardiac rehabilitation context predict their use of overprotective support strategies, and whether such overprotection in turn predicts lower self-efficacy and poorer program attendance in cardiac rehabilitation patients. Research Method: Participants were 69 partner–patient dyads, mostly older adults (mean age = 65 years) in long-term relationships (M = 35 years). During the first week of a 10-week cardiac rehabilitation program in a midsized rural hospital, participants completed self-report questionnaires that were used to assess partners' attachment styles and levels of overprotection, as well as patients' health-related self-efficacy. Attendance at each session of the program was then tracked by cardiac rehabilitation staff members. Results: A moderated mediation model using bootstrapping showed that when partners were insecurely attached (high in both attachment avoidance and attachment anxiety), a mediational model held, such that more insecure partner attachment predicted more extensive use of overprotective support strategies, which in turn predicted lower patient self-efficacy for exercise and less-frequent program attendance. Implications: Implications for training support partners in more-effective support strategies are discussed. Impact and Implications: This article identifies support partners' attachment insecurity as a potential risk factor for offering overprotective support in a cardiac rehabilitation context; demonstrates that partners' attachment styles are associated with their use of support strategies, which in turn are associated with important patient outcomes, assessed via self-report (self-efficacy) and objective measures (program attendance); and responds to a call to better understand the dynamics of more-effective versus less-effective partner support strategies, as a means to improve training. Awareness of these dynamics may help to train partners in use of more effective support strategies, and/or help buffer patients when they receive less-effective forms of support.