Theodoric of Isenburg-Büdingen (German: Diether von Isenburg-Büdingen) (1412 - 7 May 1482) was the Archbishop of Mainz from 1459 until 1461, and again from 1475 until 1482.

Theodoric was a son of Count Theodoric I of Isenburg-Büdingen. Early in childhood he was sent into a religious life. He was educated in Cologne and later Erfurt. In 1427 he became a member of the Cathedral of Mainz, in 1434 a rector in Erfurt, and in 1453 a cathedral vicar. In 1456 the cathedral chapter of Trier elected John II of Baden against Theodoric. On 18 June 1459 Theodoric was elected the Archbishop of Mainz with a clear majority over Adolph of Nassau-Wiesbaden-Idstein, however the Pope never confirmed Theodoric. In 1461 he went to Nuremberg for Imperial and Papal reform, and its recommendations earned Theodoric the wrath of both the Emperor Frederick III and Pope Pius II.

Theodoric refused to cease reforms in the church, and thus Pius II declared Adolph of Nassau the Archbishop of Mainz, beginning the Mainzer Feud. The archdiocese was wracked by violent warfare. On the night of 28 October 1462 Adolph captured the city of Mainz, killed 400 citizens and had another 400 including Johannes Gutenberg exiled, and revoked its town charter and status as an Imperial City. However Theodoric did not leave the city until 1463. When Adolph died in 1475 he recommended Theodoric be his successor, and on 9 November that year he was again elected. The reformist Pope Sixtus IV confirmed him. In 1477 Theodoric donated the University of Mainz to the city.

Theodoric died in Aschaffenburg in 1482 and was buried in Mainz Cathedral.

Preceded by:

Theodoric of Isenburg-Büdingen

Succeeded by:

Theodoric Schenk of Erbach Archbishop of Mainz
1459 - 1461
Adolph II of Nassau-Wiesbaden-Idstein
Adolph II of Nassau-Wiesbaden-Idstein Archbishop of Mainz
1475 - 1482
Adalbert III of Saxony