Introduction: Although strong scientific evidence of the efficacy and effectiveness of treatment-as-prevention (TasP) is available, full endorsement of the "Undetectable = Untransmittable" (U = U) and "zero-risk" messages could be improved. Increasing knowledge about HIV transmission, prevention and treatment is a critical component of care efforts. The study assessed knowledge of HIV transmission and prevention strategies, and the perceived accuracy of the slogan U = U among sexual and gender minorities (SGM) in Brazil. Methods: Cross-sectional web-based survey targeting adult SGM living in Brazil (2021-2022) recruited on social media and dating apps. We used the 12-item HIV Knowledge Assessment (HIV-KA) questionnaire to assess HIV knowledge, three items of which address pre-exposure prophylaxis (PrEP), post-exposure prophylaxis and TasP. Perceived accuracy of the U = U slogan was assessed with the question: "With regards to HIV-positive individuals transmitting HIV through sexual contact, how accurate do you believe the slogan U = U is?". We a priori grouped the study population into three mutually exclusive groups: people living with HIV (PLHIV), HIV negative and HIV unknown. We used logistic regression models to assess factors associated with high HIV knowledge and perception of the U = U as completely accurate. Results: Of 50,222 individuals accessing the questionnaire, 23,981 were included: 5071 (21.0%) PLHIV, 17,257 (71.5%) HIV negative and 1653 (6.9%) HIV unknown. The proportion of participants with high knowledge was significantly higher for PLHIV and HIV negative (48.1% and 45.5%, respectively) compared to 26.1% of HIV unknown. More PLHIV perceived U = U as completely accurate (80.4%), compared to 60.0% of HIV negative and 42.9% of HIV unknown. HIV knowledge correlates with perceived accuracy of the U = U slogan across all groups. Higher HIV knowledge was associated with higher income and education regardless of HIV status. Among HIV-negative participants, PrEP awareness and use were associated with higher knowledge and accurate perception of the U = U slogan. Conclusions: Our findings show that HIV knowledge and perceived accuracy of U = U are strongly correlated, that knowledge differs according to HIV status, and that poor socio-economic is linked to poor knowledge among SGM from Brazil. Educational strategies regarding TasP, U = U and zero risk targeting socio-economically vulnerable populations are urgent in Brazil.