<p>Although cancer patients have a high burden of mental illness, it is less clear whether patients with mental illness experience a higher-than-expected cancer risk. This systematic review investigated the associations between a wide range of types of mental illness and subsequent cancer risk. Five electronic databases were searched for articles published from database initiation to October 24, 2024. Studies using nationwide Nordic register data to conduct a prospective analysis of the association between mental illness diagnosis and cancer incidence were included. When data from three or more countries were available for a given type of mental illness, random effects meta-analyses were performed; otherwise, narrative syntheses were conducted. A total of 7,767 non-duplicate articles were screened, of which 20 were included. Meta-analyses showed no association between schizophrenia diagnosis and overall cancer risk; however, a high level of heterogeneity was detected between studies. Individuals with schizophrenia appeared to have a higher risk of breast cancer (pooled standardized incidence ratio [SIR]: 1.28, 95% confidence interval [CI]: 1.08-1.52), and lower risks of rectal cancer (pooled SIR: 0.75, 95% CI: 0.59-0.95) and prostate cancer (pooled SIR: 0.53, 95% CI: 0.48-0.59), compared to individuals in the general population. A higher risk of overall cancer was observed for mood, anxiety, and stress-related disorders, as well as autism and intellectual disability, in multiple studies; however, the results differed by cancer types and sex. In summary, these findings suggest an altered risk of cancer among individuals with mental illness living in Nordic countries. Further research is needed to enable the development of targeted cancer prevention strategies to reduce mental health disparities in cancer protection.</p>