Objective While men who have sex with men (MSM) are disproportionately affected by Peru's overlapping HIV and STI epidemics, there are few data on how partnership-level and network-level factors affect STI transmission in Peru. We explored partnership-level and network-level factors associated with gonorrhoea/chlamydia (Neisseria gonorrhoeae and/or Chlamydia trachomatis (NG/CT)) and/or syphilis infection among MSM in Peru. Methods We present the results of a cross-sectional secondary analysis of MSM (n=898) tested for syphilis and NG/CT infection as part of the screening process for two STI control trials in Lima, Peru. Participants completed questionnaires on demographics, sexual identity and role, characteristics of their three most recent sexual partners (partner sexual orientation, gender, role, partnership type, partner-specific sexual acts) and 30-day sexual network characteristics (number of sexual partners, partnership types, frequency of anal/vaginal intercourse). Participants were tested for syphilis and urethral, rectal and oropharyngeal NG/CT. Differences in network characteristics were analysed with χ 2 and Kruskal-Wallis tests. Results Approximately 38.9% of participants had a new STI diagnosis (syphilis (rapid plasma reagin ≥16): 10.6%; NG/CT: 22.9%; syphilis-NG/CT coinfection: 5.4%). Condomless anal intercourse (CAI) was not significantly associated with an STI diagnosis. Gay-identified participants with exclusively homosexual networks had a higher prevalence of STIs (47.4%) than gay-identified MSM with only heterosexual/bisexual partners (34.6%, p=0.04), despite reporting fewer sexual partners (any partners: 2, 1-4 vs 3, 2-6; p=0.001; casual partners: 1, 0-3 vs 2, 1-4; p=0.001) and more stable partnerships (1, 0-1 vs 0, 0-1; p=0.003) in the last month. Conclusions Network size and the number of casual sexual partners were associated with NG/CT infection among MSM in Peru. Despite reporting fewer sexual risk behaviours (smaller network size, more stable partnerships, less CAI), MSM with homosexual-only sexual networks had a higher prevalence of NG/CT and syphilis. These findings suggest network composition among MSM in Peru plays an important role in the risk for STI acquisition.