More than a decade ago, the preventive effect of antiretroviral therapy for HIV, known as "TasP" (see box), was scientifically demonstrated: an HIV-positive person on treatment with an undetectable viral load cannot transmit the virus. The objective of this article is to assess the level of knowledge about TasP among men who have sex with men in 2021, to describe their characteristics according to their self-reported HIV status, and to identify the factors associated with this knowledge. The data are drawn from the 2021 Rapport au sexe (Eras) survey, a cross-sectional, anonymous, self-administered, and voluntary online survey. Among the 14,706 respondents included in the analysis—residing in France and having had at least one sexual encounter with a man in their lifetime—60.5% are aware of TasP, 92.4% among those who are HIV-positive, and 58.2% among those who are HIV-negative or unaware of their HIV status. Logistic regressions show, with varying degrees depending on respondents’ HIV status, that a low level of education, a financial situation perceived as difficult, low health literacy, or not identifying as gay were associated with lower awareness. Conversely, living in an urban area, being part of the gay community, or using HIV-related healthcare services were positively associated with knowledge of TasP. Continuing public awareness campaigns about TasP is essential, whether through nonprofit, community, or healthcare channels, as well as general public channels, with the goal of improving knowledge about HIV and reducing HIV-related stigma.