BACKGROUND: Unintended pregnancy reflects the existence of unprotected sex. Understanding factors associated with unintended pregnancy among HIV-positive women is very important to design strategies for the prevention of further transmission and infection with the virus. However, there is scarce information in this regard. Given the degree of HIV prevalence among women and the current antiretroviral therapy scale up in Ethiopia, it is important to understand factors associated with un-intended pregnancy in order to prevent mother to child HIV transmission (MTCT). METHODS: An institution-based cross-sectional study design was employed. The sample size was 353; all HIV/AIDS sero-positive reproductive age group (15-49) women having any pregnancy history after their diagnosis and having started HAART were included and simple random sampling was used to select study participants. Data collection period was from March 9 to April 13, 2019. RESULTS: The prevalence of unintended pregnancy among the participants was 40.9%. In the multivariate logistic regression, unemployment (AOR, 3.36; 95% CI: 1.55-7.26), not being knowledgeable on MTCT and prevention of MTCT (PMTCT) (AOR, 3.18; 95% CI: 1.92-5.24), and having had no discussion on reproductive health issues (AOR, 1.83; 95% CI: 1.09-3.07) are factors significantly associated with unintended pregnancy occurrence among HIV-positive women on antiretroviral therapy. CONCLUSION AND RECOMMENDATION: The prevalence of unintended pregnancy among the women in the study is high. To avoid unintended pregnancies, HIV-infected women need access to effective family planning services and risk reduction discussions during routine care visits.