Afonso III (rare English alternatives: Alphonzo or Alphonse), or Affonso (Archaic Portuguese), Alfonso or Alphonso (Portuguese - Galician) or Alphonsus (Latin), the Bolognian (Port. o Bolonhês), the fifth King of Portugal (5 May 1210 in Coimbra – 16 February 1279 in Alcobaça, Coimbra or Lisbon) and the first to use the title King of Portugal and the Algarve, from 1249. He was the second son of King Afonso II of Portugal and his wife, Urraca, princess of Castile; he succeeded his brother, King Sancho II of Portugal, who was removed from the throne on 4 January 1248.

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 elder brother Sancho. He lived mostly in France, where 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 1248 after his brother's death. To ascend the throne, he abdicated from the county of Boulogne and later (1253) divorced Matilda.

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 comprising 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.

Afonso showed extraordinary vision for the time. Progressive measures taken during his kingship include: representatives of the commons, besides the nobility and clergy, were involved in governance; the end of preventive arrests such that henceforward all arrests had to be first presented to a judge to determine the detention measure; and fiscal innovation, such as negotiating extraordinary taxes with the mercantile classes and direct taxation of the Church, rather than debasement of the coinage. This led to his excommunication by the holy see and possibly precipitated his death, and his son Dom Dinis's premature rise to the throne at only 18 years old.

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 the Algarve should be Castilian, not Portuguese, which led to a series of wars between the two kingdoms. 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.