Persons who inject drugs (PWID) may be at risk of acquiring HIV and sexually transmitted infections (STIs) from risky sexual practices and elevated disease prevalence within their drug injection and sexual networks. We conducted a personal (egocentric) network study of young PWID (aged 18-30) from the Chicago metropolitan area. Logistic regression with generalized estimating equations evaluated associations between individual and network factors and sexual behaviors. Of 162 participants, 116 (71.6 %) were non-Hispanic White and 135 reported on 314 sexual network members. Multiplexity-having network members with overlapping roles as injection and sexual partners-was associated with more condomless vaginal sex (aOR 5.55; 95 % CI 1.62-19.0) and anal sex (aOR 6.79; 95 % CI 2.49-18.5) and less exchange sex among women (aOR 0.12; 95 % CI 0.03-0.40), adjusting for sociodemographic and sexual network characteristics. The contribution of individual and sexual network factors to HIV/STI transmission among young PWID warrants further research.