Arnulf I of Flanders (died March 27, 965), known as Arnulf the Great, was count of Flanders from 918 to 965.

Arnulf was the son of count Baldwin II of Flanders. He was named after his distant ancestor, Saint Arnulf of Metz; this was intended to emphasize his family's descent from the Carolingian royal house.

Arnulf greatly expanded Flemish rule to the south, taking all or part of Artois, Ponthieu, Amiens, and Ostravent. He exploited the conflicts between Charles the Simple and Robert I of France, and later those between Louis IV and his barons.

In his southern expansion Arnulf inevitably had conflict with the Normans, who were trying to secure their northern frontier. This led to the 942 murder of the Duke of Normandy, William Longsword, at the hands of Arnulf's men.

The Viking threat was receding during the later years of Arnulf's life, and he turned his attentions to the reform of the Flemish government.

He married Adela of Vermandois.

Arnulf's eldest son and heir Baldwin III of Flanders died in 962, so Arnulf was succeeded by Baldwin's infant son, Arnulf II of Flanders.