PURPOSE Evidence suggests an increased long-term risk of major psychiatric disorders (MPDs) in childhood and adolescent cancer survivors (CACSs). However, definitive conclusions regarding such associations and whether such associations vary for different types of cancers remain unclear. METHODS Using a nationwide data set from 2001 to 2011, we enrolled CACSs and likewise randomly selected individuals without cancer from the general population (1:10 ratio) who were matched to the CACSs with regard to demographic data. We investigated eight organ system–related cancers. The primary outcomes were the risks of seven MPD diagnoses: autism spectrum disorder (ASD), attention-deficit/hyperactivity disorder (ADHD), schizophrenia, bipolar disorder (BD), major depressive disorder (MDD), obsessive-compulsive disorder (OCD), and post-traumatic stress disorder. RESULTS CACSs (n = 5,121; mean age = 9.08 years) showed increased risks of six MPD diagnoses than controls (n = 51,210), with results as follows (in descending order): ASD (hazard ratio [HR], 10.42; associated 95% CI, 4.58 to 23.69), ADHD (HR, 6.59; 95% CI, 4.91 to 8.86), BD (HR, 2.93; 95% CI, 1.26 to 6.80), MDD (HR, 1.88; 95% CI, 1.26 to 2.79), OCD (HR, 3.37; 95% CI, 1.33 to 8.52), and post-traumatic stress disorder (HR, 6.10; 95% CI, 1.46 to 25.54). CACSs also showed earlier ages at diagnoses of ADHD, schizophrenia, MDD, and OCD than controls. The risks of MPD diagnoses vary according to specific cancer types/categories. Brain cancer and lymphatic/hematopoietic tissue cancer were associated with the greatest number of MPD diagnoses (ie, each was associated with six diagnoses). In addition, ASD and ADHD were associated with most organ system–related cancers (ie, each was associated with five categories). CONCLUSION We found that CACSs were at higher risks of MPD diagnoses than controls. Follow-up care should include psychosocial interventions focusing on early signs of mental health problems and early interventions in this high-risk group.