In this study, we investigated the role of dyadic planning for health-behavior change. Dyadic planning refers to planning health-behavior change together with a partner. We assumed that dyadic planning would affect the implementation of regular pelvic-floor exercise (PFE), with other indicators of social exchange and self-regulation strategies serving as mediators. In a randomized-controlled trial at a German University Medical Center, 112 prostatectomy-patients with partners were randomly assigned to a dyadic PFE-planning condition or one of three active control conditions. Questionnaire data were assessed at multiple time points within six months post-surgery, measuring self-reported dyadic PFE-planning and pelvic-floor exercise as primary outcomes and social exchange (support, control) and a self-regulation strategy (action control) as mediating mechanisms. There were no specific intervention effects with regard to dyadic PFE-planning or pelvic-floor exercise, as two active control groups also showed increases in either of these variables. However, results suggested that patients instructed to plan dyadically still benefited from self-reported dyadic PFE-planning regarding pelvic-floor exercise. Cross-sectionally, received negative control from partners was negatively related with PFE only in control groups and individual action control mediated between self-reported dyadic PFE-planning and PFE for participants instructed to plan PFE dyadically. Longitudinally, action control mediated between self-reported dyadic PFE-planning and pelvic-floor exercise for all groups. Findings provide support for further investigation of dyadic planning in health-behavior change with short-term mediating effects of behavior-specific social exchange and long-term mediating effects of better self-regulation.