Chlamydia trachomatis infection (CTI) is the most frequent sexually transmitted infection in western countries. Its prevalence in undocumented immigrants, a rapidly growing vulnerable population, remains unknown. We aimed to document the prevalence of CTI and associated factors at the primary health care level. This cross-sectional study included all undocumented immigrants attending a health care facility in Geneva, Switzerland. Participants completed a questionnaire and were tested for CTI by PCR assay. Three-hundred thirteen undocumented immigrants (68.4% female, mean age 32.4 (SD 8) years) agreed to participate. CTI prevalence was 5.8% (95% CI 3.3-8.4). Factors associated with higher prevalence were age â0/00¤25 (OR 3.9, 95% CI 1.3-12.2) and having had two or more sexual partners during the precedent year (OR 4.5, 95% CI 1.5-13.7). Prevalence and associated factors for infection in this vulnerable population were comparable with other populations in Western countries. Our findings support the importance of facilitating access to existing screening opportunities in particular to individuals at higher risk.