The current study investigated how students' private in-class use of technical devices is associated with teachers' mental health. Data of 361 teachers from primary and secondary schools in Germany were assessed via online surveys. The present cross-sectional results show a positive association between burden caused by the students' private in-class use of technical devices and teachers' depression symptoms. Both were negatively linked to teachers' positive mental health (PMH) and teachers' competence in handling students' private in-class use of technical devices. In a moderated mediation analysis, the association between burden and depression symptoms was mediated by PMH. Teachers' competence moderated the link between PMH and depression symptoms. Specifically, the higher the competence, the weaker the relationship between both variables. Thus, the protective effect of teachers' competence could be especially important in persons with low PMH. Competence training in handling students' use of technical devices is discussed as a potential step that could protect teachers' mental health.