Thietmar (also Dietmar or Dithmar; 25 July 975 – 1 December 1018), Prince-Bishop of Merseburg from 1009 until his death, was an important chronicler recording the reigns of German kings and Holy Roman Emperors of the Ottonian (Saxon) dynasty. Two of Thietmar's great-grandfathers, both referred to Liuthar, were the Saxon nobles Lothar II, Count of Stade, and Lothar I, Count of Walbeck. They were both killed fighting the Slavs at the Battle of Lenzen.
Thietmar was a son of the Saxon count Siegfried I the Older of Walbeck (d. 990) and his wife Kunigunde (d. 997), daughter of Henry I the Bald, Count of Stade (House of Udonids). His father fought with Margrave Odo against Duke Mieszko I of Poland at the 972 Battle of Cedynia. At the time of Thietmar's birth, his family sided with the Ottonian duke Henry II of Bavaria ("the Wrangler") in his uprising against his cousin Emperor Otto II. Later, a balance was achieved; Siegfried became burgrave at Möckern and his brother Count Lothair of Walbeck served as margrave of the Northern March from 983 until his death in 1003.