Thietmar I, Dietmar I or Ditmar I may refer to:
Dietmar I (archbishop of Salzburg) (died 907) Thietmar, Count of Merseburg (died 932), sometimes numbered Thietmar I Thietmar, Margrave of Meissen (died 979), sometimes numbered Thietmar I () Dietmar I, bishop of Verden () Dietmar I, abbot of Niederaltaich Abbey () Thietmar I, abbot of Helmarshausen Abbey ( 1058–1068) Dietmar I, abbot of Metten Abbey () Dietmar I, abbot of Seitenstetten Abbey ()