Afonso III of Portugal (English Alphonzo), or Affonso (Archaic Portuguese), Alfonso or Alphonso (Portuguese-Galician) or Alphonsus (Latin), the Bolognian (Port. o Bolonhês) or the Brave (Port. o Bravo), the fifth king of Portugal (May 5, 1210 in Coimbra – February 16, 1279 in Alcobaça, Coimbra or Lisbon) and the first to use the title King of Portugal and Algarve, since 1249. He was the second son of King Afonso II of Portugal and his wife, Urraca of Castile; he succeeded his brother, King Sancho II of Portugal in 1247.

As the second son of King Afonso II of Portugal, Afonso was not expected to inherit the throne, which was destined to go to his brother Sancho. He lived mostly in France, were he married Matilda, the heiress of Boulogne, in 1238, thereby becoming Count of Boulogne. In 1246, conflicts between his brother, the king, and the church became unbearable. Pope Innocent IV then ordered Sancho II to be removed from the throne and be replaced by the Count of Boulogne. Afonso, of course, did not refuse the papal order and marched to Portugal. Since Sancho was not a popular king, the order was not hard to enforce; he was exiled to Castile and Afonso III became king in 1247. To ascend the throne, he abdicated from the county of Boulogne and later (1253) divorced Matilda. In 1253 he married Brites (Beatrix), an illegitimate daughter of King Alfonso X of Castile. Determined not to commit the same mistakes as his brother, Afonso III paid special attention to what the middle class composed of merchants and small land owners had to say. In 1254, in the city of Leiria, he held the first session of the Cortes, a general assembly, comprised of the nobility, the middle class and representatives of all municipalities. He also made laws intended to restrain the upper classes from abusing the least favoured part of the population. Remembered as a notable administrator, Afonso III founded several towns, granted the title of city to many others and reorganized public administration.

Secure on the throne, Afonso III then proceeded to make war with the Muslim communities that still thrived in the south. In his reign the Algarve became part of the kingdom following the capture of Faro—Portugal thus becoming the first Iberian kingdom to complete its Reconquista. Following his success against the Moors, Afonso III had to deal with a political situation arising from the borders with Castile. The neighbouring kingdom considered that the newly acquired lands of Algarve should be Castilian, not Portuguese, which led to a series of wars between the countries. Finally, in 1267, a treaty was signed in Badajoz, determining that the southern border between Castile and Portugal should be the River Guadiana, as it is today.

Afonso's first wife was Matilda II of Boulogne, daughter of Renaud, Count of Dammartin, and Ida of Boulogne. She had two sons but both died young (Roberto and an unnamed one). He divorced Matilda in 1253 and in the same year married Beatrix of Castile, illegitimate daughter of Alfonso X, King of Castile, and Maria de Guzman.