This retrospective observational study conducted in Necker Hospital for Sick Children, France (January 2018–June 2020) evaluated a potential temporal association between admissions for suicide behaviours in children and adolescents and the national COVID-19 lockdown (March–May 2020). During the study period, 234 patients were admitted for suicide behaviours (28% male; mean age 13.4 years). Using Poisson regression, we found a significant decrease in the incidence of admissions for suicide behaviour during the lockdown (adjusted incidence rate ratio: 0.46; 95% CI 0.24 to 0.86). This association might result from reduced help-seeking and decreased hospital admission rates during the lockdown, as well as cognitive and environmental factors. Further multicentre studies should be conducted to confirm these findings and investigate whether a compensatory rise in admissions for suicide behaviour occurred in the postlockdown period.