José António Carlos de Seixas (Portuguese: [ˈkaɾluʃ ˈsɐj.ʃɐʃ, -ˈsej-]; 11 June 1704 – 25 August 1742) was a pre-eminent Portuguese composer of the 18th century. An accomplished virtuoso of both the organ and the harpsichord, Seixas succeeded his father as the organist for Coimbra Cathedral at the age of fourteen. In 1720, he departed for the capital, Lisbon, where he was to serve as the organist for the royal chapel, one of the highest offices for a musician in Portugal, a position which earned him a knighthood. Much of Seixas' music rests in an ambiguous transitional period from the learned style of the 17th century to the galant style of the 18th century.