OBJECTIVES: Sexual stigma affecting MSM in Nigeria may be an important driver of HIV and other sexually transmitted infections (STIs), but potential mechanisms through which this occurs are not well understood. This study assessed the contributions of suicidal ideation and sexual risk behaviors to causal pathways between stigma and HIV/STIs. DESIGN: Data were collected from the TRUST/RV368 Study, a prospective cohort of 1480 MSM from Abuja and Lagos, Nigeria. METHODS: Participants enrolled from March 2013 to February 2016 were classified into three stigma subgroups based on a latent class analysis of nine stigma indicators. Path analysis was used to test a model where disclosure led to stigma, then suicidal ideation, then condomless sex with casual sex partners, and finally incident HIV infection and/or newly diagnosed STIs, adjusting the model for age, education, having had female sex partners in the past 12 months, and sex position. Both direct and indirect (mediational) paths were tested for significance and analyses were clustered by city. RESULTS: As stigma increased in severity, the proportion of incident HIV/STI infections increased in a dose-response relationship (low: 10.6%, medium: 14.2%, high 19.0%, P = 0.008). All direct relationships in the model were significant and suicidal ideation and condomless sex partially mediated the association between stigma and incident HIV/STI infection. CONCLUSION: These findings highlight the importance of the meaningful integration of stigma-mitigation strategies in conjunction with mental health services as part of a broader strategy to reduce STI and HIV acquisitions among Nigerian MSM.