The focus on mental health outcomes in sport, including the ways in which mental health can be protected and promoted, has become a major international priority for all sports, including the recreational sports system. The aim of this paper is to outline a systems theory of mental health care and promotion that is specific to needs of the recreational sport system so that context-specific, effective policies, interventions, and models of care can be articulated and tested. Based on general systems theory, we offer a preliminary theory and accompanying postulates that outline the general principles that explain mental health in recreational sports. Namely: one of the purposes of the recreational sport system is to protect and promote the mental health of all involved; the recreational sport system is comprised of components (individuals, groups, organisations, communities, society); the role and function of those components vary by context; the purpose of the system is achieved through coordinated and reciprocal relationships between its components; and, the system should be regulated from within to ensure that mental health is protected and promoted. We have also outlined the ways in which the subcomponents of the system interact, their potential influence on mental health in recreational sports, and the ways in which they may be regulated. This information provides a theoretical foundation upon which research, programming, and policy can be based to protect and promote the mental health of all involved in recreational sports.