In 2016–2017, a large Swedish hospital introduced a mobile team of asylum social workers to work with asylum-seeking and undocumented patients. Based on interviews with the asylum social workers and the theoretical concepts of jurisdiction and professional discretion, this study explores how they experienced their work and developed their specialist competence and strategies for health social work with migrants. The findings show that they not only relieved staff and supported patients and relatives but also systemised the knowledge about health social work with migrants and created a professional specialty with a socially, rather than medically, defined group of patients.