In countries characterized by a high degree of decentralization, the limits between their main administrative units – that is, their ‘internal borders’ – with jurisdiction over protected natural areas prove critical for their management. This article analyses a national park split by just such an internal border, its objective being to determine the extent to which this border compromises the classic notion of a national park and how it impacts its management. The case study described is that of the Sierra de Guadarrama National Park, located in the centre of Spain, straddling the autonomous regions or communities of Castile and León and the Community of Madrid. The study exploits qualitative techniques, specifically semi-structured interviews with prominent actors in this territory, which allow us to analyse their perceptions of the national park and the implications of the border between the two autonomous communities in which it lies. Our research shows that the effects of internal borders can be similar to those of a border between two States, and that in decentralized countries the concept of transboundary protected areas is valid at the intra-state level. Additionally, the important political role played by national parks at different scales is verified as is the fact that cooperation between Spain’s autonomous communities remains weak.