Arnulf I of Flanders (c.890-March 27, 965), known as Arnulf the Great, was the third count of Flanders.

Arnulf was the son of count Baldwin II of Flanders and Ælfthryth, daughter of Alfred the Great. 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.

In 934 he married Adele of Vermandois, daughter of Herbert II of Vermandois. Their children were:
1. Baldwin III of Flanders
2. Luitgard, married Wichmann, Count of Ghent
3. Egbert, died 953
4. Elftrude, married Siegfried, Count of Guines

Arnulf made his eldest son and heir Baldwin III of Flanders co-ruler in 958, but Baldwin died untimely in 962, so Arnulf was succeeded by Baldwin's infant son, Arnulf II of Flanders.

Through his descendants Matilda of Flanders and Henry I of England, he was an ancestor to the present-day British royal family including Elizabeth II of the United Kingdom of Great Britain and Northern Ireland.