Introduction Data on the sexual behavior among people living with human immunodeficiency virus (PLHIV) dwelling at HIV prevalent setting located at the periphery of Ethiopia is lacking. Therefore, this study was designed to investigate sexual practice of patients following their antiretroviral therapy (ART) service and factors affecting their behavior. Materials and methods A facility based cross-sectional study design was employed to assess risky sexual practice and associated factors among HIV positive adults attending ART clinics in Gambella town, Southwest Ethiopia. Risky sexual practice is defined as a custom of getting in at least one of the following practices such as condom-unprotected sex with any partner, having two or more sexual partners and practicing casual sex in the last three months. A total of 352 randomly selected clients were interviewed by using a structured questionnaire. The multivariable logistic regression model was used to examine the association between covariates and the outcome variable. Results Majority of the study participants were engaged at least in one of the risky sexual practices (79.8% confidence interval (CI): 75.3% - 83.9%). The multivariable analysis found that the odds of risky sexual practice were higher among individuals who use substances (‘khat’ users (AOR: 3.82, 95%CI:1.30–11.22), smoke cigarette (AOR:4.90, 95%CI:1.19–12.60), consume alcohol (AOR: 2.59, 95%CI:1.28–5.21)); those who never discuss about safe sex with their partner/s (AOR: 2.21, 95%CI:1.16–4.21); those who have been in attachment for longer duration (more than four years) with their partner (AOR: 3.56, 95%CI: 1.32–9.62); and groups who desire to bear children in their future life (AOR: 3.15, 95%CI:1.40–7.04) as compared to their respective comparison groups. Conclusions A significant number of participants were engaged at least in one of the risky sexual practices which potentially result in super infection by a new or/and drug resistant viral strain/s, and onward transmission of the virus. Thus, an HIV intervention program which focuses on the identified factors has to be implemented to mitigate risk of unsafe sexual behavior of this population group and move towards ending the HIV/Acquired Immunodeficiency Syndrome (AIDS) epidemic.