Objective: Despite growing appreciation of how close relationships affect health outcomes, there remains a need to explicate the influence romantic partners have on health behavior. In this paper, we demonstrate how an established model of behavior change—the theory of planned behavior (TPB)— can be extended from an individual level to a dyadic (couple) model to test the influence that relationship partners have on a key determinant of health behavior—behavioral intentions. Methods: Two hundred romantic couples (400 individuals) completed TPB measures regarding physical activity for themselves and their romantic partner as well as a measure of relationship quality. Results: Above and beyond the individual-level TPB predictors of behavioral intentions (i.e., attitudes, subjective norms, and perceived behavioral control), the romantic partner’s perceived behavioral control (PBC) regarding physical activity predicted each individual’s behavioral intentions and moderated the influence of each individual’s PBC on his or her own behavioral intentions. Additionally, the romantic partner’s perceptions of each individual’s TPB measures predicted each individual’s behavioral intentions to be physically active. Quality of the relationship also moderated some partner influences on individuals’ intentions. Conclusions: This paper provides a roadmap for integrating a dyadic framework into individual-level models of behavior change. The findings suggest that data from both partners and relationship quality are important to consider when trying to understand and change health-related behavior such as physical activity. The results broaden the potential applications of the TPB as well as our understanding of how romantic partners might influence important health-related practices.