Britonia (which became Bretoña in Galician and Spanish) is the name of a Romano-British settlement on the northern coast of the Iberian peninsula at the time of the Anglo-Saxon invasion of Britain. The area is roughly that of the northern parts of the modern provinces of A Coruña and Lugo in the autonomous community of Galicia, Spain.

Britonia was established in the Germanic Kingdom of the Suebi, in Gallaecia, northwestern Hispania, in the late 5th and early 6th centuries AD by Romano-Britons. Britonia is therefore similar to Brittany, in Gaul (present-day France), by being settled by expatriate Britons at roughly the same time. However, unlike in Brittany, the Celts settling in the Iberian Britonia were soon assimilated and completely lost their original language.

The Britons may have occupied a pre-existing hill fort or castro. Gallaecia had earlier been inhabited by the Gallaeci peoples, before the arrival of the Germanic Suebi.

Some modern toponyms across Galicia and Asturias prove the existence of Brythonic influence on the region. Examples include Bretelo in Ourense, Bertoña in A Capela and Barro, or Playa de Bretones in Asturias.

What little is known of Britonia is deduced from its religious history - which is a very uncertain guide as to how long it retained a Celtic linguistic and cultural character. The British settlements were recognised at the First Council of Lugo in 569 and a separate bishopric established, on territory split off from the then Metropolitan Archdiocese of Lugo. Maeloc was nominated Bishop of Britonia and signed the acta at the Second Council of Braga in 572.

The establishment of the episcopal see of the Britons in Gallaecia was more probably not because of a migration, but only because a group of Christians, led by their bishop, must have taken refuge in a place near the coast of Lugo, where they would establish and organize a personal episcopal see, which later reached a territorial demarcation. For this reason the diocese was mentioned in the "Suevo Parish" as Ad sedem Britonorum ecclesiae quae sunt intro Britones una cum Monasterio Maximi et quae sunt in Asturiis. Established in Britonia, the capital of the diocese, regardless of its origin and provenance, its bishops appear in conciliar documents from the 6th century on. For example, Mailoc is among those who participated in the II Bracarense Council held in the year 572, and as its headquarters "was erected shortly before ... Mailoc signed last as least ancient". His successors attended other councils in Toledo and Braga : Errnerico participated in the III of Toledo signing, in 589, as bishop Laniobrense; Metopio attended the next one in 633; at VII, Sonna, who was already consecrated in 646 and who sent the priest Materico to the following council, in 653. In 675 Bishop Bela participated in the III Bracarense Council and then the title of Britonian appears for the last time, since Brandila and Suniagisido, who attended the XIII and XVI Councils of Toledo in the years 683 and 693, sign as Laniobrense bishops.(...)"

The British Celtic settlements^{[citation needed]} were quickly integrated and their adherence to Celtic rite lasted only until the Fourth Council of Toledo in 633 decreed the now so-called Visigothic or Mozarabic rite as the standard liturgy of Hispania.

The diocese was suppressed in 716. The line of (errant?) bishops of Britonia nevertheless existed at least until 830, when the area was attacked by the Vikings; it may have continued as late as the Council of Oviedo in 900.