The Treaty of Montgomery recognises Llywelyn ap Gruffudd as Prince of Wales, but only as a vassal of King Henry III.

The Treaty of Montgomery was an Anglo-Welsh treaty signed on 29 September 1267 in Montgomeryshire by which Llywelyn ap Gruffudd was acknowledged as Prince of Wales by King Henry III of England. It was the only time an English ruler recognised the right of a ruler of Gwynedd over Wales. Llywelyn's grandfather Llywelyn the Great had previously laid claim to be the effective prince of Wales by using the title "Prince of Aberffraw, Lord of Snowdon" in the 1230s, after subduing all the other Welsh dynasties. Likewise Llywelyn's uncle, Dafydd ap Llywelyn, claimed the title of Prince of Wales during his reign from 1240 to 1246. However, Llywelyn's supremacy in the late 1260s forced recognition of his authority in Wales by an English Crown weakened by internal division.

Llywelyn ap Gruffudd, also known as Llywelyn II and Llywelyn the Last, was Prince of Gwynedd, and later was recognised as the Prince of Wales from 1258 until his death at Cilmeri in 1282. Llywelyn was the son of Gruffudd ap Llywelyn ap Iorwerth and grandson of Llywelyn ab Iorwerth, and he was one of the last native and independent princes of Wales before its conquest by Edward I of England and English rule in Wales that followed, until Owain Glyndŵr held the title during his rebellion of 1400–1415.

Henry III, also known as Henry of Winchester, was King of England, Lord of Ireland and Duke of Aquitaine from 1216 until his death in 1272. The son of John, King of England, and Isabella of Angoulême, Henry acceded to the throne when he was only nine in the middle of the First Barons' War. Cardinal Guala Bicchieri declared the war against the rebel barons to be a religious crusade and Henry's forces, led by William Marshal, defeated the rebels at the battles of Lincoln and Sandwich in 1217. Henry promised to abide by the Great Charter of 1225, a later version of Magna Carta (1215), which limited royal power and protected the rights of the major barons. Henry's early reign was dominated first by William Marshal, and after his death in 1219 by the magnate Hubert de Burgh, Earl of Kent. In 1230 the King attempted to reconquer the provinces of France that had once belonged to his father, but the invasion was a debacle. A revolt led by William Marshal's son Richard broke out in 1232, ending in a peace settlement negotiated by the Catholic Church.