Introduction: An out-of-season increase in respiratory syncytial virus (RSV) incidence was observed in Portugal from June 2021 onwards, revealing a continuing surge in cases throughout 2021/2022 autumn/winter. We aimed to describe this out-of-season epidemic and define its epidemic period, by analysing RSV incidence from week 40 of 2020 (2020-W40) to week 18 of 2022 (2022-W18).Material and Methods: Surveillance...