This article presents findings from a case study that examined how internationalization at one Swedish upper secondary school could be understood in relation to building internal capacities for school improvement. This article used a case study methodology in which aspects of capacity building, and internal capacities constituted the theoretical point of departure. Data consisted of interviews with the principal and teachers, observations, and field notes. The article argues that internationalization at the upper secondary school supported capacity building, and it is evident that internationalization is an internal capacity. It is also apparent that the principal's leadership is distributed and vital to the school's success in terms of internationalization and school improvement. Above all, internationalization serves not only as an external form of collaboration but also as an internal capacity at the school for leadership, professional development, and internal collaboration. It is evident that these internal capacities are intertwined.