OBJECTIVES In Switzerland, universal health insurance does not cover any routine testing for sexually transmitted infections (STIs), not even in individuals at high risk, and extra-genital swabbing is not standard of care. We compared STI prevalence in a multicentre prospective observational cohort of multi-partner women with/without sex work and evaluated associated risk factors. MATERIALS AND METHODS Between January 2016 and June 2017, we offered free STI testing to women with multiple sexual partners (three or more in the previous 12 months), with follow-up examinations every 6 months. We used multiplex polymerase chain-reaction testing (for Neisseria gonorrhoeae, Chlamydia trachomatis, Trichomonas vaginalis, Mycoplasma genitalium) for pooled swabs (pharynx, urethra/vagina, anus), and antibody tests for human immunodeficiency virus (HIV) and Treponema pallidum at every visit, and for hepatitis B and C at baseline. RESULTS We screened 490 female sex workers (FSWs), including 17 trans women, and 92 other multi-partner women. More than half reported a steady partner. Previously undiagnosed HIV was found in 0.2% vs 0.0%, respectively, and T. pallidum antibodies in 5.9% vs 0.0%. STIs requiring antibiotic treatment comprised: active syphilis 1.2% vs 0.0%; N. gonorrhoeae 4.9% vs 0.0%; C. trachomatis 6.3% vs 5.4%, T. vaginalis 10.4% vs 0.0%; M. genitalium 6.7% vs 6.5%. One in four FSWs vs one in nine other women had one or more of these STIs at baseline. 15.8% vs 3.8% had a history of hepatitis B, 45.5% vs 22.8% had no immunity (HBs-AB <10 IU/l). Two FSWs had hepatitis C virus antibodies (0.4%) without concurrent HIV infection. Non-condom-use (last three months) for anal/vaginal sex was not associated with STIs. Independent risk factors were group sex (adjusted odds ratio [aOR] 2.1, 95% confidence interval [CI] 1.1–4.0), age less than 25 (aOR 3.7, 95% CI 1.6–8.9), and being active in sex work for less than 1 year (aOR 2.7, 95% CI 1.3–5.3). CONCLUSION HIV and HCV do not appear to pose a major public health problem among FSWs in Switzerland, whereas vaccination against HBV should be promoted. FSWs showed high rates of STIs requiring treatment to reduce transmission to clients and/or steady partners. FSWs should be offered low-cost or free STI screening as a public health priority.