In the 1970s, the concept of sport addiction appeared in scientific literature, warning of the addictive properties of exercise when taken to extremes. Appearing in over 6500 peer-reviewed articles in Google Scholar from 1979 to 2017, this construct is of interest to the fields of mental health and sport sociology as it provides a heuristic case to consider the conditions which allow for a category-in-the-making to gain meaning despite its absence from leading classification systems. Using Hacking's framework of ecological niches, this review of literature provides a critical examination of “sport addiction” and aims to investigate the driving forces and the means by which social actors from the scientific community negotiate the landscape and boundaries of this emerging disorder. The results highlight the prominence of psychology in the diffusion of the construct and the reticence of the medical world to legitimize it as a mental health category.