BackgroundThe bidirectional relationship between human immunodeficiency virus (HIV) and psychiatric illnesses is well documented. Misinformation about HIV transmission and prevention is associated with high rates of HIV-related risky behaviours, and therefore, HIV infection risk.AimTo assess basic HIV transmission knowledge in psychiatric patients.SettingOutpatient psychiatric clinic at Tara Psychiatric Hospital, Johannesburg, South Africa.MethodsA cross-sectional, quantitative study was conducted employing a self-administered HIV knowledge questionnaire, the 18- item HIV knowledge questionnaire (HIV-KQ18). Consent, demographic, and clinical profile information were obtained from participants meeting the selection criteria.ResultsThis study indicated a mean knowledge score of 12.6 (69.7%) out of 18, and therefore good knowledge. The highest HIV-KQ18 mean scores were found in patients with personality disorders (78.9%), anxiety disorders (75.6%) and bipolar and related disorders (71.1%). Participants with schizophrenia, depressive disorders and substance use disorders had scores ranging between 66.1% and 69.4%. Statistically significant differences in knowledge were evident based on age, marital status, level of education and employment status. Interestingly, participants who used substances had higher average basic HIV transmission knowledge scores compared to those who did not use substances.ConclusionGood overall HIV transmission knowledge was found in this population, albeit lower than in the general population. Statistically, correlates were found between psychiatric diagnosis, substance use, age, marital status, level of education, and employment status and basic level of HIV knowledge.ContributionHIV knowledge remains lower in psychiatric patients than in the general population, with correlates between demographic and clinical factors, calling for psychoeducation efforts to take all these into consideration.