Theoderich (or Theoderich von Treyden) (died 15 June 1219) was the second known missionary in Livonia after Saint Meinhard, the first Bishop of Livonia. He was previously a Cistercian monk working as a priest in Turaida (1191–1202), the first abbot of Daugavgrīva monastery (1202–1211), and appointed Bishop of Estonia during 1211–1219 by Albert of Buxhoeveden, the Bishop of Riga. He had apparently worked in missionary activities in Estonia already in 1191. What little is known about Theoderich's life, is recorded in several contemporary documents and the Chronicle of Henry of Livonia.