William Farel (Guillaume Farel, 1489-1565) was a French evangelist, and a founder of the Reformed Church in the cantons of Neuchâtel, Berne and Geneva, Switzerland. He is most often remembered for having persuaded John Calvin to remain in Geneva in 1536, and for persuading him to return there in 1541, after their expulsion in 1538. Together with Calvin, Farel worked to train missionary preachers who spread the Protestant cause to other countries, and especially to France.

Farel was a fiery preacher and an energetic critic of the Roman Catholic Church. In the earliest years of the Reformation in France, he was a pupil of the pro-reform Catholic priest, Jacques Lefevre d'Etaples. While working with Lefevre in Meaux, he came under the influence of Lutheran ideas and became an avid promoter of them. He was forced to flee to Switzerland because of controversy that was aroused by his writings against the use of images in Christian worship.