Background Public health nurses play an important role in promoting mental health to adolescents including the immigrants in school. However, a few studies have looked at either adolescent immigrants and mental health, or public health nurses and mental health. There seems to be a lack of studies combining both, to examine the public health nurse experience in promoting mental health to adolescent immigrants. Aim The study explored public health nurses’ experiences with mental health promotion for adolescent immigrants in lower secondary and high school. Method An exploratory qualitative study taking a hermeneutic approach was conducted using three focus groups of thirteen public health nurses, with at least four per group from three municipalities in Western Norway. The data was collected using focus group interviews and analyzed using a reflexive thematic analysis. It was important for the researcher, a public health nurse, and a first-generation immigrant, to reflect on her pre-understanding. Findings Three themes emerged: i) Striving to understand adolescent immigrants` mental health, ii) Different strategies for promoting mental health, and iii) Perceived barriers to promoting mental health. Conclusion While existing studies suggest that adolescent immigrants rarely seek help, the findings reveal that there seems to be rather a delay in seeking help due to language acquisition and trust, which takes time to develop. The study contributes to knowledge about strategies that can strengthen PHNs’ role as supporters for immigrant adolescents at school, uncover challenges PHNs may encounter, and inform relevant policies on public health nurses` mental health promotion work towards adolescent immigrants.