Mental health is a worldwide public health concern, as can be seen from the WHO's comprehensive mental health action plan 2013-2020 which was adopted by the 66th World Health Assembly. According to the Mental health commission of Canada (2012), one in five Canadians will personally experience a mental illness in their lifetime, and the WHO shows that mental illness represents the second most prevalent risk of morbidity after heart disease. Physical activity certainly provides an answer to this problem. Physical activity has been shown to improve physical health but it is also one of the most natural and accessible means to improve mental health. The aim of the present article is to propose a biopsychosocial model on the basis of a literature review on the psychological benefits of physical activity. In view of the findings we assume that physical activity increases mental well-being and optimal mental health as opposed to poor mental health. Hence, physical activity provides a state of well-being that enables individuals to realize their own potential, and that helps to cope with the normal stresses of life or adversity. The model certainly opens the way for research and new hypothesis, but it also aims at the promotion of the benefits of physical activity on psychological well-being for optimal mental health.