The Frisii were 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 AD) the Frisii and the related Chauci, Saxons, and Angles inhabited the Continental European coast from the Zuyder Zee to south Jutland. All of these peoples shared a common material culture, and so cannot be defined archaeologically.

The Frisii were bordered on the south by Germans who would later coalesce into the Frankish confederation in the 3rd century. On the east they were bordered by the Ampsivarii who lived at the mouth of the Ems until AD 58, at which time the Chauci expelled them and gained a border with the Frisii.

The Chauci and other tribes to the east would 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 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.