Afonso IV of Portugal (English: Alphonzo), or Affonso (Archaic Portuguese), Alfonso or Alphonso (Portuguese-Galician) or Alphonsus (Latin), (February 8, 1291 – May 28, 1357), known as the Brave (Portuguese: o Bravo), was the seventh king of Portugal from 1325 until his death. He was the only legitimate son of Dinis of Portugal by his wife Elizabeth.

Afonso, born in Lisbon, was his father's only legitimate son and the rightful heir to the Portuguese throne. However, he was not, according to several sources, Dinis' favourite son; his half-brother, the illegitimate Afonso Sanches, enjoyed full royal favour. From early in life, the notorious rivalry led to several outbreaks of civil war. On January 7, 1325, Afonso's father died and he became king, taking full revenge on his brother. His rival was sentenced to exile in Castile, and stripped of all the lands and fiefdoms donated by their common father. Afonso Sanches, however, did not sit still. From Castile, he orchestrated a series of attempts to usurp the crown for himself. After a few failed attempts at invasion, both brothers signed a peace treaty, arranged by the Queen Isabella.

In 1309, Afonso IV married princess Beatrice, daughter of King Sancho IV of Castile by his wife Maria de Molina. The first-born of this union, princess Maria of Portugal, married King Alfonso XI of Castile in 1328, at the same time that Afonso IV's heir, Peter, was promised to another Castilian princess, Constance of Penafiel. These arrangements were imperiled by the ill will of Alfonso XI of Castile, who was, at the time, publicly mistreating his wife. Afonso IV was not happy to see his daughter abused, and started a war against Castile. Peace arrived four years later, with the intervention of princess Maria herself. A peace treaty was signed in Seville in 1339 and, in the next year, Portuguese troops played an important role in the victory of the Battle of Rio Salado over the Marinids Moors in October 1340.

As king, Afonso IV is remembered as a soldier and a valiant general, hence the nickname the Brave. But perhaps his most important contribution was the importance he gave to the Portuguese navy. Afonso IV granted public funding to raise a proper commercial fleet and ordered the first maritime explorations. The Canary Islands (today a part of Spain) were discovered during his reign.