Background: We aimed to understand the relationships of the weather as biometeorological and hospital admissions due to common mental and behavioural disorders in a national setting in recent years. Methods: This is an ecological study. Ten percent of daily hospital admissions from the included hospitals (n = 1618) across Germany that were available between 1 January 2009 and 31 December 2011 (n = 5 235 600) were extracted from Statistisches Bundesamt, Germany. We identified F01-F51 mental ad behavioural disorders by International Classification of Diseases version 10 as the study outcomes. Daily weather data from 64 weather stations covering 13 German States including air temperature, humidity, wind speed, cloud cover, radiation flux and vapour pressure were obtained and generated into physiologically equivalent temperature (PET). We then plotted two-way fractional-polynomial regression. Results: For the most subtypes, the highest admissions were recorded in spring. There were small peaks in autumn or late winter for a few subtypes as well. Admissions of delirium peaked when PET was at 0 °C. Admissions of personality disorders peaked at the coldest – when PET was at − 10 °C. Admissions of schizophrenia and nonorganic sleep disorder peaked when PETs were between 0 and −10 °C while admissions of eating disorders dropped when PETs were above 10 °C. Admissions of depression and anxiety disorder did not vary much across PETs. Moreover, admissions of reaction to stress and dissociate disorder peaked when PETs were between 0 and 10 °C as well. Conclusions: More medical resources could have been needed for mental health on days when PETs were <10 °C than on other days.