Objectives: To assess knowledge and concern about monkeypox, acceptability of behavioural changes to reduce transmission risk, and willingness to be vaccinated among gay, bisexual and queer-identifying men and non-binary people. Design: National, online cross-sectional survey with convenience sample, August-September 2022. Participants were recruited through community organisation promotions, online advertising, and direct email invitations. Participants: Eligible participants were gay, bisexual or queer; identified as male (cisgender or transgender) or non-binary; aged 16 years or older; and lived in Australia. The completion rate was 70.76% (2287/3232 of those who started the survey). Main outcome measures: Participant characteristics; knowledge and concern about monkeypox; recognition of monkeypox symptoms and transmission routes; vaccination history; acceptability of behavioural changes to reduce monkeypox risk, and willingness to be vaccinated. Results: Of 2287 participants, most participants were male (2189/2287; 95.71%) and gay (1894/2287; 82.82%), nearly all had heard about monkeypox (2255/2287; 98.60%), and most were concerned about acquiring monkeypox (1461/2287; 64.42%). Most of the 2268 undiagnosed participants identified skin lesions (2087; 92.02%), skin rash (1977; 87.17%), and fever (1647; 72.62%) as potential symptoms, and prolonged and brief skin-to-skin contact as potential ways to acquire monkeypox (2124, 93.65%; and 1860, 82.01% respectively). The most acceptable behavioural changes were reducing or avoiding attendance at sex parties (1494; 65.87%) and sex-on-premises venues (1503; 66.40%), and having fewer sexual partners (1466; 64.64%). Most unvaccinated and undiagnosed participants were willing to be vaccinated (1457/1733; 84.07%). Conclusions: People at risk of monkeypox should be supported to adopt acceptable risk reduction strategies and seek vaccination as supply increases.