Abstract Men-who-have-sex-with-men (MSM) carry a disproportionate burden of sexually transmitted infections across Europe. Health planners require sexual health needs assessment data to respond appropriately. In addition, surveillance of risk and precaution behaviours in this population enables evaluative judgements of policy responses. The European MSM Internet Survey (EMIS-2017) aimed to repeat the pan-European online survey on male homosexual behaviour conducted in 2010, both to update information required to plan and monitor health promotion interventions and consolidate harmonisation of existing behavioural surveillance approaches. Our study team collaborated with more than 200 partners from academia, public health and civil society across 50 countries. Starting from our previous EMIS-2010 questionnaire, partners engaged in three rounds of iterative survey development and piloting until the final content was agreed. Transfer to an online survey application was followed by further pre-testing before translation into 32 additional languages, final testing and sign-off. The survey was available to complete online in 33 languages simultaneously from 18 October 2017 to 31 January 2018. Ten international MSM dating mobile apps were paid to send short promotional messages, and national partners promoted the survey via at least another 272 websites and social media accounts. Real-time monitoring of responses facilitated targeted advertising by country and recruitment source. Ultimately 144,305 cases were submitted of which 137,358 (95.2%) were eligible for inclusion. EMIS-2017 demonstrated the feasibility of multi-country online MSM surveying with public funding. Meaningful involvement of a large number of collaborators in the survey design and its execution ensured successful recruitment. Careful design, piloting and presentation ensured the survey was acceptable and had authority and perceived community benefit. Partners in 38 countries have already been supplied with a national database of 100 or more respondents for national analysis, while the study team has focussed on international comparisons among 137,358 respondents in 50 countries.