Juan de Valdés (c.1500 – August 1541) was a Spanish religious writer.
He was the younger of twin sons of Fernando de Valdés, hereditary regidor of Cuenca in Castile, where Valdés was born. He has been confused with his twin brother Alfonso (a courtier of Charles V, Holy Roman Emperor, who attended Charles's coronation in Aachen in 1520 and was Latin secretary of state from 1524). Alfonso died in 1532 at Vienna.
Juan, who probably studied at the University of Alcalá, first appears as the anonymous author of a politico-religious Diálogo de Mercurio y Carón, written and published about 1528. A passage in this work may have suggested Don Quixote's advice to Sancho Panza on appointment to his governorship. The Diálogo attacked the corruptions of the Roman Church; hence Valdés, in fear of the Spanish Inquisition, left Spain for Naples in 1530.
In 1531 he removed to Rome, where his criticisms of papal policy were condoned, since in his Diálogo he had upheld the validity of Henry VIII's marriage with Catherine of Aragon. On 12 January 1533 he writes from Bologna, in attendance upon Pope Clement VII From the autumn of 1533 he made Naples his permanent residence, his name being Italianized as Valdésso and Val d'Esso. Confusion with his brother may account for the statement (without evidence) of his appointment by Charles V as secretary to the viceroy at Naples, Don Pedro de Toledo; there is no proof of his holding any official position, though Curione (in 1544) writes of him as "cavalliere di Cesare." His house on the Chiaja was the centre of a literary and religious circle; his conversations and writings (circulated in manuscript) stimulated the desire for a spiritual reformation of the church.