Aims To estimate the prevalence of burnout risk among nurses during the peak of the first wave of the COVID-19 pandemic in Belgium and to identify risk groups and protective and risk factors. Background Nurses are at high risk of burnout, and this can have negative consequences for them, patients and health care systems. The pandemic may have changed their working conditions and increased their risk of burnout. Methods The risk of burnout was assessed through the Maslach Burnout Inventory Scale. Information on socio-demographic and working conditions during the pandemic was also collected. We obtained 4552 respondents through convenience sampling. Results A high risk of burnout was found in 70% of respondents. The main risk factors of burnout were the lack of personal protective equipment, changes in perceived workload and working with COVID-19 patients. Conclusions An uneven workload for nurses is an underlying problem during the COVID-19 pandemic and a significant risk factor for their burnout. The decreased workload is a risk factor for burnout as important as increased workload and repeated exposure to COVID-19.