This study explores the relationship between temperature and the number of aggressive incidents and coercive interventions in the years 2007–2019 in six psychiatric hospitals in the south of the Germany with a total of 1007 beds. The number of aggressive incidents among 164 435 admissions was significantly higher on ‘heat days’ (≥30°C). Furthermore, there was a dose–response relationship between the number of aggressive incidents and increasing temperature. In contrast, the number of coercive interventions was not related to temperature. Considering the background of global warming, rising temperature could result in more frequent aggressive behaviour during in-patient treatment of psychiatric patients.