OBJECTIVES: Syphilis and HIV epidemics overlap, yet little is known about combined network and behavioural factors that drive syphilis-HIV coinfection. Our study objective was to assess network contexts and sexual behaviours associated with syphilis-HIV co-infection and monoinfection among a particularly vulnerable subgroup: young Black men who have sex with men (YBMSM). To achieve this objective, we examined factors associated with coinfection by each subgroup as classified by syphilis-HIV infection status: (A) HIV monoinfected, (B) syphilis monoinfected and (C) neither syphilis infected nor HIV infected. In addition, we further identified the factors that are associated with HIV infection or syphilis monoinfection. METHODS: Data were collected from a sample of 365 YBMSM, aged 16-29 years, recruited through respondent-driven sampling between 2014 and 2016, in two cities with large HIV epidemics: Houston, TX, and Chicago, IL. We conducted a series of multinomial logistic regression models to predict coinfection, HIV monoinfection and syphilis monoinfection as a function of network and sexual behavioural factors. RESULTS: Coinfection was associated with having network members who are coinfected or HIV infected within one's social network. Syphilis monoinfection was associated with a higher number of social venues attended, and HIV monoinfection was associated with having more condomless top partners. CONCLUSION: Public health interventions that address the diagnosis and treatment of syphilis infection and ensure that those with syphilis are being tested for HIV may be promising in limiting the synergy of syphilis-HIV infections in onward transmission. Advancing HIV and syphilis prevention efforts in high-prevalence networks may allow prioritisation of limited resources.