OBJECTIVES: To examine changes in the occurrence of physical sex with non-steady partners among men who have sex with men (MSM) in Belgium during the first weeks of the COVID-19 lockdown and associations with sociodemographic factors, sexual practices, drug, alcohol and pre-exposure prophylaxis (PrEP) use. A secondary objective was to explore changes in PrEP use and the need for PrEP follow-up. METHODS: A cross-sectional online survey. The questionnaire was available in Dutch, French and English, between April 10 and 27 (2020), and disseminated via sexual health and lesbian, gay, bisexual, trans, queer or intersex organisations throughout Belgium. Eligibility criteria included being 18 years or older, not being exclusively heterosexual and living or being born in Belgium. RESULTS: The sample included 694 MSM. Physical sex with non-steady partners decreased from 59.1% to 8.9% during the first weeks of the lockdown. Those who had sex with non-steady partners were significantly more likely to be HIV positive, to use PrEP or to have engaged in sexual practices such as group sex, chemsex and sex work before the lockdown, compared with their counterparts. Among those who used PrEP before the lockdown, 47.0% stopped using PrEP, 19.7% used event-driven PrEP and 33.3% used daily PrEP during the lockdown. Almost two-thirds of PrEP users had a PrEP care appointment in the weeks before the lockdown and a minority received follow-up elsewhere or online. Some PrEP users had concerns regarding their follow-up. CONCLUSIONS: MSM in our survey substantially reduced sexual contact with non-steady partners during the first weeks of the COVID-19 lockdown, suggesting that the risk for HIV and STI transmission in this period was low. We recommend ensuring access to sexual health services, such as HIV testing and follow-up for PrEP for the small group having multiple sex partners and engaging in sexual practices such as chemsex, or group sex, even in times of a pandemic threat.