This is the "Philip IV of Spain in Brown and Silver" by Diego Velázquez. The King of Spain, Philip IV, was normally shown in fairly sombre clothing, so the unusual splendour of his costume here suggests that this work was made to celebrate something particular.
In 1632, he wore a similar outfit for an important ceremony in which the Cortes of Castile pledged an oath of allegiance to his son and heir, Baltasar Carlos. His sleeveless jacket and breeches are richly embellished with silver thread, and his hat is decorated with delicate feathers.
Velázquez’s signature on the petition Philip holds identifies his position as court painter. The artist rarely signed his works, only those he considered important. Philip appears as a young man, his impassive and assertive expression conveying his power.
Philip IV of Spain in Brown and Silver hangs in The National Gallery on Trafalgar Square, London — free to enter. Point your phone at any artwork there and audioguide.london plays a free audio guide in six languages — no app download needed.