Due to certain risky behaviors, persons with severe mental illnesses are potentially prone to develop human immunodeficiency virus (HIV) infection more than people in general. Therefore, it is often argued that mentally ill persons should be considered as high risk groups, and so, they should go for HIV testing. For combating dual illnesses of HIV and mental illness together, there are several barriers that appear. Studies have been conducted to explore risk factors of HIV infection and HIV prevalence among mentally ill persons, and also on the probability that a mentally ill person get HIV tested compared with general population. However, hardly ever, studies have looked into barriers of HIV testing among mentally ill persons in mental healthcare settings. On this background, we have reviewed literature to explore barriers in performing HIV testing of a mentally ill person in mental healthcare settings. An important areas were found to act as barriers that added stigma due to dual illness of mental health and HIV, gender, type of facility, and its preparedness. Another barrier is the validity of the informed consent of a mentally ill person, which is required for a test. There are several conflicting situations on how to deal with this dual illness. The study recommends further exploratory researches on the matter and developing standard HIV testing protocol in mental healthcare settings.