Background: Worldwide, there is limited epidemiologic evidence on the seroprevalence of undiagnosed chronic viral infections including HIV, hepatitis B virus (HBV) and hepatitis C virus (HCV) infections among patients with severe psychiatric disorders. To our knowledge, this is the first study to explore and compare undiagnosed seroprevalence rates of HIV, HBV, and HCV infections among patients with severe psychiatric disorders. Method: In this study, we included a random sample of 309 patients with severe psychiatric disorders selected by systematic sampling technique. We used a structured clinical interview for DSM-IV (SCID) to confirm the diagnosis of severe psychiatric disorders among the participants. Binary and multivariable logistic regression models, adjusting for the potential confounding factors was used to explore the potential determinants of chronic viral infections. Result: The prevalence estimates of HIV infection among patients with severe psychiatric disorders in this study (3.24%) was roughly 3 times the estimated population prevalence of HIV infection in Ethiopia (1.1%). This study showed that the prevalence rates of HBV and HCV infections among patients with severe psychiatric disorders were 4.85 and 1.29%, respectively. Our results also showed that among patients with chronic viral infections, HIV, HBV and HCV, 76.92, 60, 80, and 75% respectively were undiagnosed. Regarding associated factors, the presence of chronic viral infection was found to be significantly associated with the age of the participants (ranging between 30 and 40 years) after adjusting for the possible confounding factors [AOR = 3.95 (95%CI.18-13.17)]. Conclusion: Even though the prevalence estimates of HIV (3.24%), HBV (4.85%), and HCV (1.29%) infections were high among patients with severe psychiatric disorders, the majority of them remained undiagnosed. HBV was found to be the commonly undiagnosed infection (4 out of 5) followed by HCV (3 out of 4) and HIV (6 out of 10). The present study provided evidence of a significant association between the age of the participant (between 30 and 40 years) and chronic viral infections in patients with severe psychiatric disorders. Increasing the awareness of psychiatry professionals and early screening, as well as interventions of chronic viral infections among patients with severe psychiatric disorders are imperative.