Female sex workers (FSWs) are at increased risk of HIV and face significant barriers to clinic-based HIV testing, including provider stigma and privacy constraints. HIV self-testing (HIVST) has been proven to significantly increase HIV testing among FSWs. Less is known, however, about how FSWs make meaning of oral-fluid HIV self-tests, and the unintended ways they use and understand this novel technology. From October 2016 to March 2017, we conducted 61 in-depth interviews with FSWs ( = 31) in Kampala, Uganda. Eligible participants were: female, ≥18 years, exchanged sex for money or goods, and had not recently tested for HIV. We used inductive coding to identify emerging themes and re-arranged these into an adapted framework. Unintended ways FSWs described self-testing included as a means to test others, to bolster their reputation as a health-conscious sex worker, and to avoid bearing witness to suffering at health facilities. Unintended meanings ascribed to self-testing included misunderstandings about how HIV is transmitted (via saliva versus blood) and whether self-tests also test for other infections. HIVST can increase FSWs' knowledge of their own HIV status and that of their sexual partners, but messaging and intervention design must address misunderstandings and misuses of self-testing. ClinicalTrials.gov identifier: NCT02846402.