Otgar (died 21 April 847), also spelled Odgar or Otger, was the archbishop of Mainz from 826 until his death.

Upon his return he brought some of the relics of Justinus and built Saint Justin's Church in Höchst to house them.

[1] In 838, Bishop Otgar translated the relics of Saint Aurelius of Riditio from the chapel of Saint Nazarius at Hirsau to the Church of St. Aurelius at Hirsau Abbey and formally consecrated the abbey church.

In 842, he even tried to prevent Louis the German from meeting Charles the Bald and allying with him against Lothair.

When Otgar finally died, Louis's hold on East Francia strengthened under his successor, Rabanus Maurus.