BACKGROUND: The 2022 multicountry mpox outbreak positioned the condition as a public health emergency of international concern. By May 2023, Brazil ranked second globally in the cumulative number of mpox cases and deaths. The higher incidence of mpox among gay and other men who have sex with men in the current mpox outbreak deepens the stigma and discrimination against sexual and gender minorities (SGM). This might worsen the structural barriers impacting access to health services, which ultimately leads to undertesting and underreporting of cases. There are no data available on mpox knowledge and stigma in Latin America. OBJECTIVE: We aimed to evaluate mpox knowledge, stigma, and willingness to vaccinate for mpox among SGM, and to describe sociodemographic and behavioral characteristics according to self-reported mpox diagnosis. METHODS: A cross-sectional, internet-based survey was conducted in a convenience sample of adults (aged >18 years) living in Brazil recruited through advertisements on dating apps, social media, referral institutions for infectious diseases websites, and mass media (October-November 2022). We compared participants' characteristics according to self-reported mpox diagnosis using chi-square test or Fisher exact test for qualitative variables and Kruskal-Wallis test for quantitative variables. RESULTS: We enrolled 6236 participants: 5685 (91.2%) were cisgender men; 6032 (96.7%) were gay, bisexual, or pansexual; 3877 (62.2%) were White; 4902 (78.7%) had tertiary education; and 4070 (65.2%) reported low or middle income. Most participants (n=5258, 84.4%) agreed or strongly agreed that "LGBTQIA+ individuals are being discriminated and stigmatized due to mpox." Mpox awareness was 96.9% (n=6044), and 5008 (95.1%) were willing to get vaccinated for mpox. Overall, 324 (5.2%) reported an mpox diagnosis. Among these, 318 (98.1%) reported lesions, 178 (56%) local pain, and 316 (99.4%) sought health care. Among participants not reporting a diagnosis, 288 (4.9%) had a suspicious lesion, but only 158 (54.9%) of these had sought health care. Compared to participants with no diagnosis, those reporting an mpox diagnosis were younger (P<.001), reported more sex partners (P<.001), and changes in sexual behavior after mpox onset (P=.002). Moreover, participants diagnosed with mpox reported more frequently being tested for HIV in the prior 3 months (P<.001), living with HIV (P<.001), currently using HIV pre-exposure prophylaxis (P<.001), and previous sexually transmitted infection diagnosis (P<.001). CONCLUSIONS: Our results point to high mpox knowledge and willingness to vaccinate among SGM in Brazil. Participants self-reporting mpox diagnosis more frequently reported to be living with HIV, STI diagnosis, and current pre-exposure prophylaxis use, highlighting the importance of an mpox assessment that includes comprehensive sexual health screenings. Efforts to decrease stigma related to mpox among SGM are necessary to avoid mpox underdiagnosis.