The sobriquet Most Faithful King (Rex Fidelissimus) was a title awarded by the Pope as spiritual head of the Catholic Church to the Portuguese monarchy.
The title remains attached to monarchs descended from whoever received the original sobriquet. The sobriquet can be awarded to either a king or a queen.
The only European monarchy that has received the sobriquet Fidelissimus was the now-defunct monarchy of Portugal. King John V of Portugal was favoured with the title of Rex Fidelissimus in 1748 by Pope Benedict XIV.