Background: A high prevalence of symptoms of mental disorders (SOMD) has been found among people living with HIV/AIDS (PLHIV). Additionally, SOMD may impact on the prevalence of high-risk health behaviours (HRB). This study investigates the relationship between SOMD and HRB in a large sample of Vietnamese HIV positive outpatients. Methods: A cross-sectional study was conducted with 400 outpatients at two HIV/AIDS clinics in Ho Chi Minh City, Vietnam, selected using a systematic sampling technique. Validated scales were used to measure SOMD, specifically symptoms of depression, anxiety, alcohol use disorder (AUD), substance use disorder (SUD) and HIV associated dementia (HAD). Participants completed a self-report questionnaire assessing HRB during the preceding 12 months including unsafe sexual practices and illicit drug use. Multivariable logistic regression models were used to evaluate associations between SOMD and HRB. Results: The majority of participants (63.5%) were male and the median age was 34.0 years. Unsafe sexual practices and illicit drug use were reported by 13.8 and 5.5% of participants. The prevalences of HAD, depression, AUD, anxiety and SUD symptoms were 39.8, 36.5, 13.3 10.5, 3.3% respectively. There was no association between SOMD and HRB either with or without adjusting for correlates of HRB, except between symptoms of SUD and illicit drug use. PLHIV who had symptoms of SUD were more likely to use illicit drugs (adjusted Odds Ratio 81.14, 95% CI 12.55-524.47). Conclusions: While the prevalence of SOMD among HIV positive outpatients was high, most SOMD were not associated with increased HRB. Only illicit drug use was predicted by symptoms of SUD. Screening PLHIV for symptoms of SUD may be useful for detecting people likely to be engaging in illicit drug use to reduce the risk of secondary disease transmission.