Arnulf I, called der Böse ("the Bad") (died July 14, 937), was duke of Bavaria from 907 until his death. He was a member of the Luitpolding dynasty.

Arnulf was the son of Margrave Luitpold of Bavaria and Kunigunde of East Francia, daughter of Berchthold I, count of Swabia. Arnulf strengthened his power through confiscation of church property, which gained him the nickname "der Böse". He refounded the duchy of Bavaria. In the stuggle for the kingship of East Francia he was a bitter opponent of Conrad of Franconia (the second husband of Arnulf’s mother Kunigunde) but later he allied himself with Conrad's successor Henry the Fowler in order to acknowledge Henry's sovereignty over Bavaria.

Arnulf was married to Judith of Friaul, daughter of Count Eberhard of Sülichgau and Gisela of Verona. Arnulf's daughter Judith married Henry I of Bavaria, brother of Otto I, Holy Roman Emperor.

Arnulf died in Regensburg in 937 and was buried in St. Emmeram.