BACKGROUND: Compared with the general female population, female sex workers (FSWs) experience a greater burden of STIs, including HIV. Consistent condom use reduces HIV risk; however, while many FSWs are aware of condom efficacy, this knowledge does not consistently translate into use. We aimed to estimate the prevalence and patterns of condomless sex by partner type at the sexual encounter level, as well as identify factors associated with condomless sex among FSWs recruited into a diary study in Zimbabwe. METHODS: We conducted a longitudinal coital diary study in site A and site B between 25 November 2020 and 30 December 2021. Pictorial diaries were developed in collaboration with FSWs. We recruited participants using snowball sampling and asked them to complete a daily diary of their sexual encounters with each partner for one month, repeated over three non-consecutive months within 12 months. The following FSW characteristics were recorded: FSW age, Key Populations (KP) Programme contact, partner type and age, condom use categorized as none, partial, or full, types of sexual activity, and violence experiences. To identify factors associated with condomless sex, we used hierarchical Poisson regression modelling. RESULTS: 404 FSWs documented 62,559 sex encounters, with 17,325 (27.7%) reported as condomless. Our adjusted analysis showed that encounters with permanent partners had the highest likelihood of being condomless (59.5%; adjusted relative risk [aRR] 3.83, 95% CI: 3.38-4.35), followed by regular partners (25.6%; aRR 1.63, 95% CI: 1.50-1.78), compared to new partners (15.0%). Compared with FSWs aged < 25 years (29.7%), those aged ≥ 35 years were less likely to have condomless sex (22.1%; aRR 0.79, 95% CI: 0.66-0.94), while sex without violence was less likely (24.1%) to be condomless than sex with violence (57.7%; aRR 2.32, 95% CI: 2.00-2.70). FSWs with the KP programme contact reported fewer condomless encounters (25.9%; aRR 0.81, 95% CI: 0.90-0.94) than those without (32.9%). CONCLUSION: Condomless sex was common and varied based on FSW age, partner type, experiencing violence during sex, and KP programme contact. These findings emphasize the need for continued condom promotion, counseling, violence mitigation, and training in condom negotiation skills especially given the heightened risk of HIV transmission among FSWs.