Jan van Leeuwen (–1378) was a lay brother of the Augustinian priory of Groenendaal and a mystical author who wrote over twenty treatises in Dutch. He was a cook and a member of the lowest social class, but his works enjoyed great popularity in his lifetime.

Life Jan was born into the lowest social class around 1300 or 1310. Nothing is known of his early life, although it has been surmised that he was a blacksmith or perhaps a cook for the duke of Brabant. He entered the hermitage at Groenendaal in 1344, a year after its founding by Jan van Ruusbroec. By his own account, he was illiterate at the time. Ruusbroec served as his confessor and his main theological teacher, whom he praises highly in his writings. He performed manual labour, working mainly as a cook, at Groenendaal for the rest of his life. He never became a canon and died in 1378.

Henricus Pomerius (died 1469) included a biography of Jan in his Latin chronicle of Groenendaal, . Jan was posthumously known as the "good cook" ().

Works Jan is the only known Christian mystical writer of his social class.: "He is thus the only known layperson of the lowest social class ever to put pen to paper in the production of a mystical corpus, and for this alone he may be considered unique in the history of medieval Christian mysticism." He wrote twenty-two mystical treatises in Middle Dutch. His earliest writings were finished after Groenendaal adopted the Augustinian rule in 1350 but before 1352. He was popular in his own lifetime, with some of his works being edited as early as 1355. A compilation of his complete works was circulating within decades of his death. His somewhat rough and "unsophisticated" writing, however, lacks a modern readership, unlike that of contemporary Rheno-Flemish mystics like Ruusbroec and Hadewijch. His writings have been considered difficult because of their lack of artifice and occasional circularity.

Themes Jan van Leeuwen is famous for his criticism of Meister Eckhart. He devoted an entire book to it, ("On Meister Eckhart's Doctrine in Which He Goes Astray"). It has generally been held in low regard. Jan was familiar with at least one of Eckhart's sermons and with the condemnation of some of Eckhart's ideas in the papal bull . Jan also mentions Eckhart in ("On the Five Kinds of Brotherhood"), ("On the Ten Commandments") and ("On a Sacramental Meal"). In his treatise ("Various Good Learnings"), he attacks Beguines, Lollards and Free Spiritists, but does not mention Eckhart. Among the things he criticises as errors are:

Jan wrote two texts on the Ten Commandments, ("The Book on God's Ten Commandments") and ("The Book on the Ten Commandments").

Manuscripts Jan's writings were edited by the brothers of Groenendaal before dissemination. A first collection of five of his works was edited in 1355. A second collection of nine—the original five plus four more—appeared towards the end of the century.

The 1355 edition is known only from two manuscripts: Deventer, Stadsarchief en Athenaeumbibliotheek, I, 55, copied at the about 1425 and Pommersfelden, Bibliothek Graf von Schönborn, 280/2881, copied by Peter von Zutphen at the in 1459. The former contains just one work while the latter, although heavily Germanized, contains all five.

The second edition is known from three copies. The earliest, from about 1400, is Brussels, Koninklijke Bibliotheek, II 138. It is an illuminated manuscript but incomplete. Only six leaves remain and some miniatures in Cape Town, National Library of  South Africa, Grey 3 c 4. The earliest complete copy is Brussels, Koninklijke Bibliotheek, IV 401, made around 1460 for Count John IV of Nassau. The last copy, also compelte, was made by Jan de Swettere, a lay brother of the in 1540 (now Brussels, Koninklijke Bibliotheek, 667).

In the years 1543–1547, Jan de Swettere also copied a collection of thirteen additional works that may be the only surviving copy of the second edition's second volume, now Brussels, Koninklijke Bibliotheek, 888–890.

List of works The following table presents Jan van Leeuwen's works in the order in which they appeared in the edited collections, with their Dutch and Latin titles and dates of composition.

Title (Dutch)Title (Latin)Date13581371–1374? after 1355before 135213521352–1353between 1353 and 1355before 135513551356 before 1352–1353 before 1355before 1355?

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