King Philip IV of Spain (1605 - 1665) was the son of Philip III and Margaret of Austria. His reign saw the decline of Spanish power. He wears the badge of the Order of the Golden Fleece on a gold chain.
Probably painted about 1656, and one of the last portraits Velázquez made of his royal patron. Many copies and versions of this portrait type exist.
From The National Gallery Podcast: Episode Forty Six, August 2010