The Gelasian is an age in the international geologic timescale or a stage in chronostratigraphy, being the earliest or lowest subdivision of the Quaternary Period/System and Pleistocene Epoch/Series.
In 2009 it was moved from the Pliocene to the Pleistocene so that the geologic time scale would be more consistent with the key changes in Earth's climate, oceans, and biota that occurred 2.58 million years ago.
The top of the Gelasian is defined magnetostratigraphically as the end of the Olduvai (C2n) chronozone, and faunally as the extinction level of the calcareous nannofossil Discoaster brouweri (base of biozone CN13).
In the regional glacial history of the Alps, this age is now called Biber.
The Gelasian is an equivalent of the Praetiglian and Tiglian Stages as defined in the Netherlands, which are commonly used in northwestern Europe.