INTRODUCTION: Patients with mental health disorders are at risk for receiving inequitable cancer treatment, likely resulting from various structural, social, and health-related factors. This study aims to assess the relationship between mental health disorders and the use of definitive treatment in a population-based cohort of those with localized, clinically significant prostate cancer. METHODS: We conducted a cohort study analysis in SEER (Surveillance, Epidemiology, and End Results)-Medicare (2004-2015). History of a mental health disorder was defined as presence of specific ICD (International Classification of Diseases)-9 or ICD-10 diagnostic codes in the 2 years preceding cancer diagnosis. Descriptive statistics were performed using Wilcoxon rank-sum and χ2 testing. Multivariable logistic regression was used to evaluate the relationship between mental health disorders and definitive treatment utilization (defined as surgery or radiation). RESULTS: Of 101,042 individuals with prostate cancer, 7,945 (7.8%) had a diagnosis of a mental health disorder. They were more likely to be unpartnered, have a lower socioeconomic status, and less likely to receive definitive treatment (61.8% vs 68.2%, P < .001). Definitive treatment rates were >66%, 62.8%, 60.3%, 58.2%, 54.3%, and 48.1% for post-traumatic stress disorder, depressive disorder, bipolar disorder, anxiety disorder, substance abuse disorder, and schizophrenia, respectively. After adjusting for age, race and ethnicity, marital status and socioeconomic status, history of a mental health disorder was associated with decreased odds of receiving definitive treatment (OR 0.74, 95% CI 0.66-0.83). CONCLUSIONS: Individuals with mental health disorders and prostate cancer represent a vulnerable population; careful attention to clinical and social needs is required to support appropriate use of beneficial treatments.