German Lutheran theologian whose mystical writings were widely circulated in Europe in the 17th century.

Arndt studied at Helmstadt, Wittenberg, Strasbourg, and Basel. In 1583 he became a pastor at Badeborn, but in 1590 he was deposed for refusing to remove pictures from his church and to discontinue the use of exorcism in Baptism. Both were considered offenses against the Calvinist concept of strict purity and simplicity. Arndt found asylum in Quedlinburg the same year and in 1599 was transferred to St. Martin's Church at Brunswick.

The principal work among his many writings, which were inspired by the mystics St. Bernard of Clairvaux, Johann Tauler, and Thomas à Kempis, is Vier Bücher vom wahren Christentum (1605–09; “Four Books on True Christianity”). Translated into most European languages and widely distributed in Arndt's time, it served as the foundation of many devotional books, both Roman Catholic and Protestant. Its publication aroused strong controversy among Lutherans. It was also a chief influence in the life of Philipp Jakob Spener (Spener, Philipp Jakob), who was a founder of Pietism, a movement that stressed simple Christian living. Arndt held that to follow orthodox doctrine was not enough and that the Christian must undergo a moral purification through righteous living and communion with God.

The opposition aroused by his book caused difficulty for Arndt in Brunswick. In 1609 he moved to Eiseleben and in 1611 to Celle, where he remained until his death.