The unusual splendour of the King's dress suggests that this work records a particular occasion, since he is usually depicted in more sombre clothes. He wears the badge of the Order of the Golden Fleece on a gold chain.
This work was probably painted soon after Velázquez's return from Italy in 1631. It is one of the few signed paintings by the artist and the principal portrait of the King of that period.
From The National Gallery Podcast: Episode Forty Six, August 2010