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Frederick II of Dießen (also known as Frederick I of Regensburg; 1005 – 1075) was a German nobleman. He is documented as bailiff (Vogt) of the Regensburg cathedral chapter in 1035. He is one of the earliest known ancestors of the Counts of Andechs.

His father was Count Frederick of Dießen (d. c. 1030), a relative of the legendary Bavarian count Rasso (d. 954), who administered the area around Dießen and Haching. His mother was Hemma, a daughter of Duke Conrad I of Swabia.

He became Domvogt of Regensburg in 1035. In 1055, he became Count in the Sempt area.

He died in 1075, as a lay brother in the Sankt Blasien Abbey in the Black Forest.

Frederick married three times:

1. Hadamut (d. 1060), a daughter of Eberhard of Eppenstein. Together, they had one daughter:
   • Haziga (c. 1040 – 1 August 1104), also known as Hadegunde, married Herman of Kastl and secondly Otto I, Count of Scheyern
2. Irmgard of Gilching. Together, they had the following children:
   • Uta, married Kuno of Rott, Count palatine of Bavaria
   • Arnold (d. after 1091), succeeded his father as Count of Dießen
   • Frederick, succeeded as Vogt of the cathedral chapter of Regensburg
   • Meinhard (d. after 1070), succeeded as Count of Gilching
   • Hemma
   • Liutgard, married Count Adalbert I of Bogen
   • Berthold, Count jure uxoris of Schwarzenburg
3. Tuta, a daughter of the Vogt Hartwig I of Regensburg. This marriage was childless.