chief founder of the Swiss Brethren, an Anabaptist movement centred on Zürich.

His humanist education at Basel, Vienna, and Paris led him gradually to oppose the conservative Swiss Reformer Huldrych Zwingli (Zwingli, Huldrych), whom he had earlier supported. The slowness of reform and Grebel's increasing impatience to create a church free from control by the Zürich city council led to his permanent break with Zwingli in 1524.

That year, with several colleagues, Grebel began to organize the Brethren as an independent, radical Anabaptist church in Zürich. The dispute over infant versus adult Baptism culminated (January 1525) in the city council's defense of infant Baptism and in its order to Grebel to cease his efforts to organize the Brethren. In the same month, however, he disobeyed the edict by performing the first adult Baptism in modern history and by setting out to win converts in nearby cities.

Despite his missionary successes, he was harassed until his death. He was imprisoned on two occasions in Zürich for a total of at least six months. Like other Brethren, Grebel believed in living simply and in rejecting violence, in spite of the warfare that raged among religious factions. His only extant writings are the 69 letters preserved at Sankt Gallen.