Theoderich (or Theoderich von Treyden) 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.
Theoderich was killed by Estonians in 1219. After his death, the title "Bishop of Estonia" was not used any more, being temporarily replaced by the "Bishop of Leal" before the Estonian territory was divided into several dioceses.
What little is known about Theoderich's life, is recorded in several contemporary documents and the Chronicle of Henry of Livonia.