Introduction: Antiretroviral therapy (ART) outcomes that include viral suppression rates are rarely reported among African prison populations. Prisoners deal with specific challenges concerning adherence to ART. We aimed to describe virological outcomes of ART in a large prison in Malawi.Methods: A cross-sectional study of ART outcomes was conducted at the Zomba Central Prison HIV clinic, Malawi, following the introduction of routine viral load monitoring. All prisoners on ART for at least 6 months were eligible for a viral load test. Patients with ≥1,000 copies/ml received adherence support for 3 months, after which a second VL sample was taken. Patients with ≥5,000 copies/ml on the second sample had virological failure and started 2nd line ART. We describe demographics and patient characteristics and report prevalence of potential- and documented virological failure. In the potential virological failure rate, those who could not be sampled after 3 months adherence support are included as virological failures. Logistic regression analysis was used to determine factors associated with potential ART failure.Results and discussion: Viral load testing was started at the end of 2014, when 1054 patients had ever registered on ART. Of those, 501 (47.5%) had transferred out to another clinic, 96 (9.1%) had died, 11 defaulted (1.0%) and 3 (0.3%) stopped ART. Of 443 (42.0%) remaining alive in care, an estimated 322 prisoners were on ART >6 months, of whom 262 (81.4%) were sampled. Their median age was 35 years (IQR 31–40) and 257 (98.1%) were male. Self-reported adherence was good in 258 (98.5%). The rate of potential ART failure was 8.0%, documented ART failure was 4.6% and documented HIV suppression 95.0%. No patient characteristics were independently associated with potential ART failure, possibly due to low numbers with this outcome.Conclusions: Good virological suppression rates can be achieved among Malawian prisoners on ART, under challenging circumstances.