Objectives: The study aimed to assess the magnitude of undiagnosed hypertension, and its associated factors among adult HIV-positive patients receiving antiretroviral therapy at Butajira General Hospital, southern Ethiopia. Methods: We applied an institutional-based cross-sectional study design at Butajira General Hospital from 1 May to 1 July 2021. We used a systematic random sampling technique to select the total number of participants. A structured interviewer-administered questionnaire was applied to collect the data (sociodemographic characteristics, clinical-related factors, and lifestyle-related factors from the study participants. Data were entered using Epi-data version 3.1 and analyzed by statistical package for social science version 25. We applied a multivariable logistic regression analysis model to identify variables significantly associated with hypertension. Results: The study comprised 388 participants with 39 years (10.6 SD) as the mean age of the participants. Of the total participants, 235 (60.6%) were female. In this study the magnitude of undiagnosed hypertension among HIV-positive patients was 18.8% (95% CI: 14.7%–23.2%). Having comorbidity of diabetes mellitus (adjusted odds ratio = 5.29, 95% CI: 2.154, 12.99), habit of alcohol drinking (adjusted odds ratio = 2.909, 95% CI: 1.306, 6.481), duration of antiretroviral therapy ⩾ 5 years (adjusted odds ratio = 3.087, 95% CI: 1.558, 6.115), and age ⩾ 40 years (adjusted odds ratio = 2.642, 95% CI: 1.450, 4.813) were factors significantly associated with undiagnosed hypertension. Conclusions and recommendations: The magnitude of undiagnosed hypertension among HIV-positive patients attending the antiretroviral therapy clinic of Butajira General Hospital is high. The findings of this study implied that HIV-positive patients attending antiretroviral therapy clinics should be monitored routinely for hypertension; especially participants aged ⩾40 years, highly active antiretroviral therapy duration ⩾5 years, having diabetes mellitus comorbidity need more attention. Primary healthcare integration is also vital to enhance the health of HIV-positive patients on antiretroviral therapy.