BACKGROUND: Men who have sex with men (MSM) and male-to-female transgender women (transwomen) are disproportionately at risk of syphilis infection in Peru. METHODS: From 2013 to 2014, MSM and transwomen seeking human immunodeficiency virus (HIV) or sexually transmitted infection (STI) testing and/or treatment were recruited into a 2-year observational cohort study to determine predictors of recently acquired syphilis infection (defined as a rapid plasma reagin [RPR] titer >/=1:16 and a reactive treponemal antibody test) in Lima, Peru. At baseline, interviewers collected sociodemographic, behavioral, and medical characteristics from participants. All cohort participants were tested for syphilis, HIV, Chlamydia trachomatis (CT), and Neisseria gonorrhoeae (NG) infection. Using cross-sectional analyses, bivariate and multivariate models were used to determine factors associated with recently acquired syphilis infection and calculate adjusted prevalence ratios. RESULTS: We recruited 401 participants, 312 MSM and 89 transwomen, with median ages of 29.0 and 32.5 years old (interquartile ranges: 23.3, 37.4 and 27.2, 39.5, respectively). The prevalence of recently acquired syphilis infection at baseline was 16.8% for MSM and 6.7% for transwomen. Among MSM and transwomen, 30.1 and 33.7% were infected with HIV, 18.6 and 24.7% were infected with CT, and 14.2 and 19.1% were infected with NG, respectively. Co-infection rates among MSM with recently acquired syphilis infection included: 44.2% with HIV, 40.4% with CT (32.7% with anal CT and 7.7% with pharyngeal CT), and 19.2% with NG (11.5% with anal NG and 7.7% with pharyngeal NG). Co-infection rates among transwomen with recently acquired syphilis infection included: 66.7% with HIV, 0% with CT, and 16.7% with anal NG. In multivariate analysis among the entire cohort, recently acquired syphilis infection was independently associated with younger age (adjusted prevalence ratio [aPR] = 0.96, 95% confidence interval [CI] = 0.93-0.99), receptive role during anal sex (aPR = 2.56, 95% CI = 1.05-6.25), prior HIV diagnosis (aPR = 1.70, 95% CI = 1.11-2.61), anal CT or NG infection (aPR = 1.69, 95% CI = 1.09-2.60), and prior syphilis diagnosis (aPR = 3.53, 95% CI = 2.20-5.68). CONCLUSIONS: We recruited a cohort of MSM and transwomen who had a high prevalence of recently acquired syphilis infection in Lima, Peru. Recently acquired syphilis infection was associated with socio-demographic characteristics, sexual risk, and sexually transmitted co-infections.