Juan de Valdés (c.1490 – August 1541) was a Spanish religious writer and Catholic reformer.
 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 in 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.[citation needed] 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 moved 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.
 His first production at Naples was a philological treatise, Diálogo de la Lengua (1535). His works entitle him to a foremost place among Spanish prose writers. His friends urged him to seek distinction as a humanist, but his bent was towards problems of Biblical interpretation in their bearing on the devout life.  Vermigli (Peter Martyr) and Marcantonio Flaminio were leading spirits in his coterie, which included the marchioness of Pescara Vittoria Colonna (April 1490; a widow since 1525 – 25 February 1547, aged 57), since 1537, and her younger widowed sister-in-law, Giulia Gonzaga, (1513; marries 1526, aged 13; a widow since 1529, aged 16 – 16 April 1566, aged 53).
 His influence was great on Ochino, for whose sermons he furnished themes. Pietro Carnesecchi, (24 December 1508 – 1 October 1567), burned by the Inquisition in 1567, who had known Valdés at Rome as "a modest and well-bred courtier," found him at Naples (1540) "wholly intent upon the study of Holy Scripture," translating portions into Spanish from Hebrew and Greek, with comments and introductions. To him Carnesecchi ascribes his own adoption of the Evangelical doctrine of justification by faith, and at the same time his rejection of the policy of the Lutheran schism. Valdés died at Naples in May 1541.
 His death scattered his band of associates. Abandoning the hope of a regenerated Catholicism, Ochino and Vermigli left Italy.  Some of Valdés's writings were by degrees published in Italian translations. Showing much originality and penetration, they combine a delicate vein of semi-mystical spirituality with the personal charm attributed to their author in all contemporary notices.  Llorente traces in Valdés the influence of Tauler; any such influence must have been at second hand.  The Aviso on the interpretation of Scripture, based on Tauler, was probably the work of Alfonso. Valdés was in relations with Fra Benedetto of Mantua, the anonymous author of Del Benefizio di Gesù Cristo Crocefisso, revised by Flaminio (reprinted by Dr Babington, Cambridge, 1855).
 The suggestion that Valdés departed from Catholic Orthodoxy about the Trinity was first made in 1567 by the Transylvanian bishop, Ferenc Dávid; it has been adopted by Sand (1684), Wallace (1850) and other nontrinitarian writers, and is countenanced by Bayle. To this view some colour is given by isolated expressions in his writings, and by the subsequent course of Ochino (whose heterodox repute rests, however, on the insight with which he presented objections).  Gaston Bonet-Maury (1842–1919) comments: "Valdés never discusses the Trinity (even when commenting on Matt, xxviii. 19), reserving it (in his Latte Spirituale) as a topic for advanced Christians; yet he explicitly affirms the consubstantiality of the Son, whom he unites in doxologies with the Father and the Holy Spirit" (Opusc. p. 145). Practical theology interested him more than speculative, his aim being the promotion of a healthy and personal piety.
 Notices of Valdés are in Sandius, Christopher (1684), Bibliotheca antitrinitariorum, Bayle and Wallace, Robert (1850), Antitrinitarian Biography. Revival of interest in him is due to
 Fuller knowledge of his career was opened up by Benjamin B. Wiffen, whose Life of Valdés is prefixed to Betts's translation of the Considerations, 1865. Discoveries have since been made in the Aulic Library, Vienna, by Edward Boehmer: