The Frisii are an ancient Germanic tribe living in the low-lying region between the Zuiderzee and the River Ems.

In the Germanic pre-Migration Period (i.e., before c. 300 CE) the Frisii and the related Chauci, Saxons, and Angles inhabite the Continental European coast from the Zuyder Zee to south Jutland.

All of these peoples share a common material culture, and so cannot be defined archaeologically.The Frisii are bordered on the south by Germans who will later coalesce into the Frankish confederation in the 3rd century.

On the east they are bordered by the Ampsivarii who live at the mouth of the Ems until CE 58, at which time the Chauci expel them and gain a border with the Frisii.The Chauci and other tribes to the east will merge to form the Saxons in the 3rd century.

Some or all of the Frisii may have joined in either or both of these confederations, but they will retain a separate identity in Roman eyes until at least 296, when they are 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 are abandoned by c. 400 due to flooding caused by a marine transgression and lie empty for a century, when changing environmental conditions again make the region habitable.

At this time, settlers repopulated the region and come to be known as 'Frisians'.

Medieval and later accounts of 'Frisians' refer to these 'new Frisians' rather than to the ancient Frisii.