Interparental interactions have an important influence on child well-being and development. Yet prior theory and research have primarily focused on interparental conflict as contributing to child maladjustment, which leaves out the critical question of how interparental positive interactions—such as expressed gratitude, capitalization, and shared laughter—may benefit child growth and development. In this article, we integrate theory and research in family, relationship, and affective science to propose a new framework for understanding how the heretofore underexamined positive interparental interactions influence children: interparental positivity spillover theory (IPST). IPST proposes that, distinct from the influence of conflict, interparental positive interactions spill over into children’s experiences in the form of their (a) experience of positive emotions, (b) beneficially altered perceptions of their parents, and (c) emulation of their parents’ positive interpersonal behaviors. This spillover is theorized to promote beneficial cognitive, behavioral, social, and physiological outcomes in children in the short term (i.e., immediately after a specific episode of interparental positivity, or on a given day) as well as cumulatively across time. As a framework, IPST generates a host of novel and testable predictions to guide future research, all of which have important implications for the mental health, well-being, and positive development of children and families.