Afonso I of Portugal (English Alphonzo or Alphonse), more commonly known as Afonso Henriques, or also Affonso (Archaic Portuguese), Alfonso or Alphonso (Portuguese-Galician) or Alphonsus (Latin version), (Guimarães, 1109?, traditionally July 25 – Coimbra, 1185 December 6), also known as the Conqueror (Port. o Conquistador), was the first King of Portugal, declaring his independence from León.

Afonso I was the son of Henry of Burgundy, Count of Portugal and Teresa of León, the illegitimate daughter of King Alfonso VI of Castile and León. He was proclaimed King on July 26, 1139, immediately after the Battle of Ourique, and died on December 6, 1185 in Coimbra.

At the end of the 11th century, the Iberian Peninsula political agenda was mostly concerned with the Reconquista, the driving out of the Muslim successor-states to the Caliphate of Córdoba after its collapse. With European military aristocracies focused on the Crusades, Alfonso VI called for the help of the French nobility to deal with the Moors. In exchange, he was to give the hands of his daughters in wedlock to the leaders of the expedition and bestow royal privileges to the others. Thus, the royal heiress Urraca of Castile wedded Raymond of Burgundy, younger son of the Count of Burgundy, and her half-sister, princess Teresa of León, wedded his cousin, another French crusader, Henry of Burgundy, younger brother of the Duke of Burgundy, whose mother was daughter of the Count of Barcelona. Henry was made Count of Portugal, a burdensome earldom south of Galicia, where Moorish incursions and attacks were to be expected. With his wife Teresa as co-ruler of Portugal, Henry withstood the ordeal and held the lands for his father-in-law.

From this wedlock several sons were born, but only one, Afonso Henriques (meaning "Afonso son of Henry") thrived. The boy followed his father as Count of Portugal in 1112, under the tutelage of his mother. The relations between Teresa and her son Afonso proved difficult. Only eleven years old, Afonso already had his own political ideas, greatly different from his mother's. In 1120, the young prince took the side of the archbishop of Braga, a political foe of Teresa, and both were exiled by her orders. Afonso spent the next years away from his own county, under the watch of the bishop. In 1122 Afonso became fourteen, the adult age in the 12th century. He made himself a knight on his own account in the Cathedral of Zamora, raised an army, and proceeded to take control of his lands. Near Guimarães, at the Battle of São Mamede (1128) he overcame the troops under his mother's lover and ally Count Fernando Peres de Trava of Galicia, making her his prisoner and exiling her forever to a monastery in León. Thus the possibility of incorporating Portugal into a Kingdom of Galicia was eliminated and Afonso become sole ruler (Dux of Portugal) after demands for independence from the county's people, church and nobles. He also vanquished Alfonso VII of Castile and León, another of his mother's allies, and thus freed the county from political dependence on the crown of León and Castile. On April 6, 1129, Afonso Henriques dictated the writ in which he proclaimed himself Prince of Portugal.

Afonso then turned his arms against the everlasting problem of the Moors in the south. His campaigns were successful and, on July 26, 1139, he obtained an overwhelming victory in the Battle of Ourique, and straight after was unanimously proclaimed King of Portugal by his soldiers. This meant that Portugal was no longer a vassal county of León-Castile, but an independent kingdom in its own right. Next, he assembled the first assembly of the estates-general at Lamego, where he was given the crown from the archbishop of Braga, to confirm the independence.

Independence, however, was not a thing a land could choose on its own. Portugal still had to be acknowledged by the neighbouring lands and, most importantly, by the Roman Catholic Church and the Pope. Afonso wedded Mafalda of Savoy, daughter of Count Amadeo III of Savoy, and sent Ambassadors to Rome to negotiate with the Pope. In Portugal, he built several monasteries and convents and bestowed important privileges to religious orders. In 1143, he wrote to Pope Innocent II to declare himself and the kingdom servants of the Church, swearing to pursue driving the Moors out of the Iberian peninsula. Bypassing any king of Castile or León, Afonso declared himself the direct liegeman of the Papacy. Thus, Afonso continued to distinguish himself by his exploits against the Moors, from whom he wrested Santarém and Lisbon in 1147 (see Siege of Lisbon). He also conquered an important part of the land south of the Tagus River, although this was lost again to the Moors in the following years.

Meanwhile, King Alfonso VII of Castile (Afonso's cousin) regarded the independent ruler of Portugal as nothing but a rebel. Conflict between the two was constant and bitter in the following years. Afonso became involved in a war, taking the side of the Aragonese king, an enemy of Castile. To ensure the alliance, his son Sancho was engaged to Dulce Berenguer, sister of the Count of Barcelona, and princess of Aragon. Finally, in 1143, the Treaty of Zamora established peace between the cousins and the recognition by the Kingdom of Castile and León that Portugal was an independent kingdom.

In 1169, Afonso was disabled in an engagement near Badajoz by a fall from his horse, and made prisoner by the soldiers of the king of León. Portugal was obliged to surrender as his ransom almost all the conquests Afonso had made in Galicia in the previous years.

In 1179 the privileges and favours given to the Roman Catholic Church were compensated. In the papal bull Manifestis Probatum, Pope Alexander III acknowledged Afonso as King and Portugal as an independent land with the right to conquer lands from the Moors. With this papal blessing, Portugal was at last secured as a country and safe from any Castilian attempts of annexation.

In 1184, in spite of his great age, he had still sufficient energy to relieve his son Sancho, who was besieged in Santarém by the Moors. He died shortly after, in 1185.

The Portuguese revere him as a hero, both on account of his personal character and as the founder of their nation.

Afonso married in 1146 Mafalda or Maud of Savoy (1125-1158), daughter of Amadeo III, Count of Savoy, and Mafalda of Albon.