José António Carlos de Seixas, (11 June 1704 – 25 August 1742), was a Portuguese composer, the son of the cathedral organist, Francisco Vaz and Marcelina Nunes.
Seixas was born in Coimbra. He succeeded his father as organist in 1718 at age fourteen, and two years later moved to Lisbon, where he gave harpsichord lessons and met Domenico Scarlatti who was living in Portugal from 1721 to 1728. It is claimed that when the king's son, Dom António, arranged for Scarlatti to give Seixas harpsichord lessons, Scarlatti replied that it was Seixas who should give him lessons. Seixas later became organist in the court chapel and Lisbon cathedral as well as court composer. He was knighted in 1738 by John V of Portugal. He married at 28 and had five children, two sons and three daughters. Seixas died of rheumatic fever.