This is the "A Knight of Saint John" painted by Rosso Fiorentino. The subject is a member of the Order of Saint John, formed in the eleventh century to care for injured crusaders. This order was known for its dual purpose: defending and protecting, as seen in the pose of this knight. His left hand clasps a sword, a symbol of military strength.
The painting is over life-size, making it larger than many of Rosso's other portraits. The portrait may have been intentionally made taller to showcase the sword hilt. The distinctive spiky fingers and extravagant vermilion hat are characteristic of Rosso's style and sense of invention.
Rosso moved to Rome in 1523, where he likely met this knight. The costume worn by the subject is not ceremonial but rather more subdued outdoor attire. This painting reflects the influence of Rosso's time in Rome, particularly with its massive scale and bulk reminiscent of Michelangelo and Sebastiano.
A Knight of Saint John 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.