Aim: To determine the associations between outdoor air pollution and hospital outpatient visits for mental disorders in China. Methods: We obtained data of 111,842 hospital outpatient visits for mental disorders from the largest hospitals of 13 cities, China, between January 01, 2013 and December 31, 2015. We collected air pollutant data including particulate matter <= 2.5 mu m in diameter (PM(2.)5), particulate matter <= 10 mu m in diameter (PM10), nitrogen dioxide (NO2), ozone (O-3) and sulphur dioxide (SO2) from China National Environmental Monitoring Centre during the same period. We conducted a time-stratified case-crossover design with conditional logistic regression models to determine the associations. Results: A 10 mu g/m(3) increase in PM2.5, PM10, NO2 and SO2 was associated with a significant increase in hospital outpatient visits for mental disorders on the current day. When stratified by age, sex and season, the effects of PM2.5 and NO2 were robust among different subgroups at lag05 days. PM10 showed positive associations in males, in cold season, and in depression patients. SO2 showed positive associations in males, in cold season, and in anxiety patients. O-3 showed positive associations in females, in warm season, and in depression patients. Nearly one sixth hospital outpatient visits for mental disorders can be attributable to NO2. Conclusions: Short-term increase in PM2.5, PM10, NO2, SO2 and O-3 concentrations was significantly associated with exacerbation of mental disorders in China as indicated by increases in hospital outpatient visits. NO2 had more serious health threat than other pollutants in terms of mental disorders. Our findings strongly suggest a need for more strict emission control regulations to protect mental health from air pollution.