OBJECTIVE: Previous studies have investigated the incidence of type 2 diabetes in individuals with psychiatric disorders, but most studies have focused on a specific psychiatric disorder or a selected sample. More population-based studies are needed to determine these associations in representative samples. We therefore aimed to determine these associations in a nationwide, register-based dynamic cohort study. RESEARCH DESIGN AND METHODS: We analyzed data from 5,005,612 adults living in Denmark between 1995 and 2018, without prior diabetes. We investigated 10 different categories of psychiatric disorders and a composite group with any psychiatric disorder. Individuals with a psychiatric disorder were compared with individuals without using multivariable-adjusted Poisson regression to estimate incidence rate ratios (IRR) of type 2 diabetes. We modeled age-specific incidence rates (IR) for individuals with and without the specific psychiatric disorder. All models were stratified by sex. RESULTS: In total, 334,739 individuals developed type 2 diabetes during follow-up. For all investigated categories of psychiatric disorders, we found increased IR of type 2 diabetes for individuals with versus those without a psychiatric disorder (IRR: men, 1.47 [95% CI 1.45-1.50]; women, 1.65 [95% CI 1.62-1.68]). When we examined age-specific IR, the largest differences were found in the younger population (<50 years). CONCLUSIONS: We found that the IR of type 2 diabetes was higher in individuals with a psychiatric disorder compared with individuals without a psychiatric disorder and particularly high in the younger people with a psychiatric disorder. New studies into the prevention and early detection of type 2 diabetes in these groups are warranted.