The purpose of this study was to assess the prevalence of depression and associated factors among people living with HIV/AIDS undergoing ART in Armed Forces Comprehensive Specialized Hospital, Addis Ababa. The samples of this study were 351 peoples, living with HIV/AIDS undergoing treatment who are repeatedly evaluated by the department of ART clinic and get anti-retroviral therapy selected through systematic random sampling method. This study employed mixed research design where much emphasis given to quantitative data.rnThe quantitative data was collected using socio demographic questioners, Beck Depression Inventory Scale, Oslo-3- Items Social Support Scale and other factors contributing to depression such as discordant HIV status between couples, disclosed their HIV status, and duration on treatment were assessed. The qualitative data were collected by semi-structured interview guide. Multiply logistic regression analyses showed that high prevalence of depression was found in the primary educational level (p=.002), monthly income (p=.001), social support factors (p=.000) and people with discordant HIV test result (p=.000) were significantly contribute to depression among people living with HIV/ AIDS undergoing antiretroviral treatment. In this study, the prevalence of depression and associated with higher risk of depressive among people living with HIV/ AIDS undergoing antiretroviral treatment in AFCSH. Hence, the Study has recommended that in the absence of well skilled Counselors and psychosocial services providers , creating validated instruments or check list or mental health assessment tools that could early assess, diagnose people with depression and facilitate a referral system for depressed clients should be given appropriate attention.