Background: Schools are described as important arenas for health promotion interventions. Despite the critical role of school nurses in health promotion, limited research specifically examines school nurses’ perceptions of providing health promotion. Aim: We aimed to describe both ratings and descriptions of school nurses' capability, opportunity, and motivation to provide health promotion beyond the individual health dialogues in Swedish schools. Method: This convergent mixed-methods study used national cross-sectional data collected in May 2023 from a web-based survey and qualitative narrative data from an open-ended question in the same survey. In total, 596 school nurses in Sweden answered the web survey, and 354 described their experiences in the open-ended question. Data were analysed and interpreted through three commonly used theoretical domains in behaviour change research: capability, opportunity, and motivation. Results: School nurses’ perceived motivation and capability to provide health promotion were generally rated as high, whilst the perceived opportunities to provide health promotion was rated as lower. Individual factors and the local work environment seemed to influence school nurses' perceived capability, opportunity, and motivation to provide health promotion. In a positive work environment, school nurses were confident in colleagues’ expectations and how to assess health promotion needs. In addition, there was a joint agenda, structure, and interprofessional collaboration in such a school, where health promotion felt important, enjoyable, and rewarding. In a negative work environment, school nurses were not expected to provide health promotion, and their time was prioritised to include other tasks primarily. Moreover, there was a lack of perceived adequate knowledge, skills, and tools to provide health promotion. Conclusion: A positive work environment for school nurses, including a joint health promotion agenda, interprofessional collaboration, and clear expectations and structure, seem to be essential for providing health promotion activities to school-aged children.