Sixt Dietrich (Xistus Theodericus) (* c. 1494 in Augsburg; † October 21, 1548 in St. Gallen) was a German composer. At the age of ten at the latest (until 1508) he was a choirboy at the cathedral choir in Konstanz (Kostnitz). On September 23, 1509 he went to the University of Freiburg and fled from there to Constance because he was in debt. There he taught music and Latin to the choir boys and was ordained deacon in 1519. In 1522 he celebrated his first Mass but due to the reformation he lost his income in 1527. After a one year period in Wittenberg (from 1540) and a short stay in Constance he returned to Wittenberg in 1544. Now residing in Constance again and two months before his death, he had to flee from there to St. Gallen because of the city capture by Charles V where he died.

Dietrich was regarded as one of those composers who transposed Dutch polyphony into the music of early Protestant services. His compositions were used by both Catholics and Lutherans.