The Frisii were an ancient Germanic tribe living in the low-lying region between the Rhine–Meuse–Scheldt delta and the River Ems, and the presumed or possible ancestors of the modern-day ethnic Dutch.

The Frisii lived in the coastal area stretching roughly from present-day Bremen to Bruges, including many of the smaller offshore islands. In the 1st century BC, Romans took control of the Rhine delta but Frisii to the north of the river managed to maintain some level of independence. Some or all of the Frisii may have joined into the Frankish and Saxon peoples in late Roman times, but they would retain a separate identity in Roman eyes until at least 296, when they were forcibly resettled as laeti  (i.e., Roman-era serfs) and thereafter disappear from recorded history. Their tentative existence in the 4th century is confirmed by archaeological discovery of a type of earthenware unique to 4th-century Frisia, called terp Tritzum, showing that an unknown number of Frisii were resettled in Flanders and Kent, likely as laeti under the aforementioned Roman coercion.

The lands of the Frisii were largely abandoned by c. 400, probably due to climatic deterioration and flooding caused by sea level rise. They lay empty for one or two centuries, when changing environmental and political conditions made the region habitable again. At that time, settlers that came to be known as 'Frisians' repopulated the coastal regions. Medieval and later accounts of 'Frisians' refer to these 'new Frisians' rather than to the ancient Frisii.