Objective: HIV disproportionately affects young men who have sex with men, and knowledge about HIV transmission is one factor that may play a role in high rate of infections for this population. This study examined racial/ethnic differences in HIV knowledge among young men who have sex with men in the USA and their correlation to condom usage errors. Design: Participants included an ethnically diverse sample of 344 young men who have sex with men screened from an ongoing longitudinal cohort study. Eligible participants were between the ages of 16 and 20?years, born male and had previously had at least one sexual encounter with a man and/or identify as gay or bisexual. This analysis is based on cross-sectional data collected at the baseline interview using computer-assisted self-interviewing (CASI) software. Setting: Chicago, IL, USA Method: We utilised descriptive and inferential statistics, including analysis of variance (ANOVA) and Tukey's post hoc analysis to assess differences in HIV knowledge by level of education and race/ethnicity, and negative binomial regression to determine whether HIV knowledge was associated with condom errors while controlling for age, education and race/ethnicity. Results: The study found that Black men who have sex with men scored significantly lower (average score?=?67%; p < 0.05) than their White counterparts (average score?=?83%) on a measure of HIV knowledge (mean difference?=?16.1%, p < 0.001). Participants with less than a high school diploma and those with a high school diploma/general education diploma (GED) only had lower knowledge scores, on average (66.4% and 69.9%, respectively) than participants who had obtained post-high school education (78.1%; mean difference?=?11.7% and 8.2%, respectively, ps < 0.05). In addition, controlling for age, race and level of education, higher HIV knowledge scores were associated with fewer condom errors (Exp B?=?0.995, confidence interval [CI]?=?0.992-0.999, p < 0.05). Conclusion: Findings stress the need for increased attention to HIV transmission-related educational activities targeting the social realities and unique risk mechanisms of young men who have sex with men.