count of Flanders (918–958, 962–965) and son of Baldwin II.

On his father's death in 918, the inherited lands were divided between Arnulf and his brother Adolf, but the latter survived only a short time, and Arnulf succeeded to the whole inheritance. His reign was filled with warfare against the Norsemen, and he took an active part in the struggles in Lorraine between the emperor Otto I and Hugh Capet.

In 958 Arnulf placed the government in the hands of his son Baldwin (Baldwin III), and the young man, though his reign was a very short one, did a great deal for the commercial and industrial progress of the country, establishing the first weavers and fullers at Ghent and instituting yearly fairs at Ypres, Bruges, and other places. On Baldwin III's death in 962 the old count, Arnulf I, resumed control and spent the few remaining years of his life in securing the succession of his grandson Arnulf II the Younger (reigned 965–988).