Leo Jud (/dʒʌd/; also Leo Juda, Leo Judä, Leo Judas, Leonis Judae, Ionnes Iuda, Leo Keller; 1482 – 19 June 1542), known to his contemporaries as Meister Leu, was a Swiss reformer who worked with Huldrych Zwingli in Zürich.

Jud was born in 1482 in Guémar, Alsace, southwest of Strasbourg. Like his later colleague Heinrich Bullinger, Jud was the son of a priest. His father, Johannes Jud, was from Guémar and his mother, Elsa Hochsang, was from Solothurn.

He studied at the Latin school in Sélestat with Martin Bucer and in 1499 he matriculated at the University of Basel, where he met Huldrych Zwingli. He began his studies in medicine but switched to theology under the influence of Thomas Wyttenbach. Leo Jud is one of the lesser-known leaders of the Swiss Reformed Church, but his influence was extensive.

Jud was ordained by the Church at Rome in 1507, and from 1507 to 1512 he was a deacon at Saint Theodore in Basel, then, from 1512 to 1518, he served as the preacher at Saint Hippolyte in Alsace. In 1519, he became Zwingli's associate at Einsiedeln (in Schwyz), where his reform-minded tendencies showed through clearly.

The 1520s were a time of great productivity for Jud. On 9 March 1522, he participated in the Affair of the Sausages during Lent, a protest against the established food mandates of the church. That summer he translated Zwingli's petition to the bishop of Constance (Konstanz) for the legitimization of clerical marriage from Latin into German.

In June 1522, Jud was selected to be pastor at Saint Peter's in Zürich (on Zwingli's recommendation), although he did not take up his post until 2 February 1523. Jud assumed his preaching duties only four days after the First Zürich Disputation.

During 1523, Jud became increasingly involved in Reformation efforts. On 7 March 1523, he was appointed the pastor of the Oetenbach nunnery in Zürich, which housed Dominican nuns. That summer he began preaching against clerical marriage prohibitions, which resulted in a group of the nuns at petitioning to be released from their vows. The same year, he drew up the baptismal rite for the Swiss Reformed Church in German, which still retained some Catholic elements to it. In September of that year, Jud preached a sermon in Saint Peter's against religious images, and the result was several acts of iconoclasm in Zürich. Jud (along with Zwingli) called for the complete removal of images, desiring the restoral of the apostolic church.

This call for iconoclasm was something characteristic of some French-speaking Swiss reformers as well, such as Pierre Viret and Guillaume Farel. Images in the church remained a point of contention between the Swiss Reformers and magistrates for another several years.