Chemsex-the use of drugs in a sexual context-has been associated with more at-risk sexual practices and substancerelated complications in men who have sex with men (MSM). To date, no study has focused on the impact of France's first coronavirus disease 2019 (COVID-19)-related lockdown on the mental health and drug/alcohol use of MSM who practice chemsex. We implemented a web-based survey of 9,488 MSM living in France in June 2020 (after the country's first COVID-19 lockdown). Specifically, we first compared the subpopulation of MSM who self-reported practicing chemsex during their most recent sexual intercourse (defined as "chemsexers") with other MSM, using five outcomes: increased 1/tobacco use, 2/alcohol use, and 3/other psychoactive drug use. 4/using psychotropic medication during the lockdown, and finally 5/psychological distress. We then analyzed the outcomes' associations with the main explanatory variable "chemsexer," after adjusting for all relevant variables. Among 7,195 MSM who had sexual intercourse with a man during the previous 6 months, 359 participants (5%) were identified as "chemsexers." Multivariable analyses showed that during the first lockdown period, chemsexers were significantly more likely than non-chemsexers to have increased their use of tobacco, alcohol, and other psychoactive substances. Chemsexers were also more likely to have used psychotropic medication and to have experienced psychological distress during the previous month. Given the ongoing COVID-19 pandemic in France and worldwide, this finding highlights the need to develop psychosocial interventions and harm reduction services for MSM chemsexers, potentially via mobile health.