Background:Personality traits, loneliness and mental health are conditions often evaluated in patients with chronic, disabling or stigmatizing conditions. These conditions contribute to the health seeking behavior and outcome of management of diseases especially in stigmatizing illnesses like HIV/AIDS. This study therefore assessed the prevalence of personality traits, loneliness and mental health among HIV/AIDS clinic attendees in a Nigerian tertiary health institution. Method: The general health questionnaire (GHQ-12), the Big Five Personality Inventory and the University of California Los Angeles (UCLA) loneliness scale, version 3, were used to assess a total of 310 HIV/AIDS clinic attendees in a Nigerian tertiary health institution for prevalence of personality traits, loneliness and mental health. Results: The various aspects of personality traits differed in their prevalence. The most prevalent was openness (27.4%), followed by neuroticism (25.5%), conscientiousness (19.0%), agreeableness (15.5%) and extraversion (12.6%). About 33.2% of the subjects indicated experiencing frequent loneliness, while 11.9% indicated severe loneliness. Furthermore 32.9% showed the presence of mental health problems as against 67.1% who indicated absence of mental health problems. Conclusion: This study revealed the prevalence of various forms of personality traits, loneliness and presence of mental health problems among the subjects.