The texts of ancient Romans called it Lake Flevo. Lake Almere is mentioned among others in a life of saints written by Anglo-Saxon Bishop Saint Boniface in 753, and a deed of gift from the town of Urk.
A number of occurrences during the Middle Ages led to the transformation of the lake to an inland sea that would be called the Zuiderzee, which are:
- rising sea levels due to global warming known as the Medieval Warm Period.
- excavation of peat by the Frisians in West Friesland near the Vlie, a river that connected lake Almere to the North Sea.
- floods such as the All Saints' Flood (1170) and St. Lucia's flood 1287.