Arnulf I of Flanders (c. 890 – March 28, 965), called 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 dynasty.

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 943 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:

* Luitgard, married Wichmann, Count of Hamaland * Egbert, died 953 * Baldwin III of Flanders * Elftrude, married Siegfried, Count of Guînes

He also had a previous daughter, Hildegard.

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.

* Flodoard * Folcwine * Lambert of Ardres * Platts, Beryl. The Scottish Hazard: Flemish Nobility and their Impact on Scotland, 1985

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