BackgroundPoor knowledge regarding STI and HIV transmission, testing and treatment among men who have sex with men (MSM) may be contributing to their disproportionate burden of STIs. However, factors which predispose MSM to having this low knowledge are less understood.AimThe aim of this study was to identify factors associated with having lower knowledge of HIV and STI transmission, testing and treatment among MSM.MethodsThe MSM Internet Survey Ireland 2015 was a self-completed online national survey available to MSM living in Ireland. Thirteen factual statements were used to assess participants’ knowledge of HIV and STI transmission, testing and treatment. Respondents were defined as having ‘lower knowledge’ if they indicated prior knowledge of fewer than 11 true statements. Multivariable-adjusted logistic regression was used to identify factors associated with having lower knowledge.Results3090 MSM completed the survey, of whom 2905 (94%) were included in this study. Thirty-six per cent (n=1055) had lower knowledge of HIV and STI transmission, testing and treatment. The factors associated with having lower knowledge included being 18–24 years of age (vs >40 years; adjusted OR (aOR) 1.98, 95% CI 1.50 to 2.61), born in Ireland (vs outside Ireland; aOR 1.62, 95% CI 1.25 to 2.10) and being out to none of their contacts (vs out to most/all; aOR 1.69, 95% CI 1.20 to 2.37). Knowledge was also lower among those who never tested for HIV (vs tested negative within last 12 months; aOR 2.32, 95% CI 1.88 to 2.86) and among MSM who never visited an MSM-specific health promotion website (vs visited website; aOR 1.81, 95% CI 1.45 to 2.25).ConclusionA range of demographic factors, sexual health variables and use of MSM-specific sexual health promotion materials are associated with low knowledge regarding HIV and STI transmission, testing and treatment among MSM in Ireland. Engagement with the main national MSM-specific sexual health promotion website was associated with higher knowledge levels.