Arnulf I, called der Böse ("the Bad" or "the Evil") (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, daughter of Berchthold I, count of Swabia. Besieged by frequent raids by the Hungarians and desperate to raise funds to finance a defence, Arnulf strengthened his power through confiscation of church lands and property, which earned him the nickname "the Bad". He re-established the duchy of Bavaria and eventually negotiated a truce with the Hungarians who thereafter largely passed though Bavaria on their raids into other German territories. Arnold vigorously resisted Emperor Conrad, Duke of Franconia (and the second husband of Arnulf's mother Kunigunde), but later submitted to Conrad's successor as King of the Germans, Henry the Fowler of Saxony, who confirmed Arnulf'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.