The Principality or Kingdom of Gwynedd is one of several successor states to Rome which emerge in 5th-century Britain during the Coming of the Saxons.

Based in northwest Wales, the rulers of Gwynedd repeatedly rise to preëminence and are acclaimed as "King of the Britons" before losing their power in civil wars or invasions.

The unitary kingdom of the usurper Gruffydd ap Llywelyn is shattered by a Saxon invasion in 1063 just prior to the arrival of the Normans, but the traditional dynasty restored by Gruffydd ap Cynan slowly recovered until Llywelyn the Great of Gwynedd was able to proclaim the Principality of Wales at Aberdyfi in 1216.

That realm lasta until the invasions of Edward I in 1283.Welsh tradition credits the founding of Gwynedd to Picts from Lothian invading the lands of the Brythonic Deceangli, Ordovices, and Gangani in the 5th century.

The sons of their leader Cunedda Wledig are said to have possessed the land between the rivers Dee and Teifi.

The true borders of the realm vary over time, but Gwynedd Proper is generally thought to comprise the cantrefs of Aberffraw, Cemais, and Cantref Rhosyr on Anglesey and Arllechwedd, Arfon, Dunoding, Dyffryn Clwyd, Llŷn, Rhos, Rhufoniog, and Tegeingl on the mountainous mainland opposite.

The modern preserved county of Gwynedd and principal area of Gwynedd are both somewhat smaller.