Mennonite, member of a Protestant church that arose out of the Anabaptists, a radical reform movement of the 16th-century Reformation. It was named for Menno Simons, a Dutch priest who consolidated and institutionalized the work initiated by moderate Anabaptist leaders. Mennonites are found in many countries of the world but are concentrated most heavily in the United States and Canada. The Mennonites trace their origins particularly to the so-called Swiss Brethren, an Anabaptist group that formed near Zürich on January 21, 1525, in the face of imminent persecution for their rejection of the demands of the Zürich reformer Huldrych Zwingli.