Robert I, called "The Magnificent" (French, "le Magnifique") for his love of finery, and also called "The Devil" was the son of Duke Richard II of Normandy and Judith, daughter of Conan I, Duke of Brittany.

When his father died, his elder brother Richard succeeded, whilst he became Count of Hiémois. When Richard died a year later, there were great suspicions that Robert had Richard murdered, hence his other nickname, "Robert le diable" (the devil). He is often mis-identified with the legendary Robert the Devil. Robert aided King Henry I of France against Henry's rebellious brother and mother, and for his help he was given the territory of the Vexin. He also intervened in the affairs of Flanders, supported Edward the Confessor, who was then in exile at Robert's court, and sponsored monastic reform in Normandy.

By his mistress, Herleva, he was father of two children:
1. the future William the Conqueror.
2. Adeliza, Countess of Aumale

After making his illegitimate son William his heir, he made a pilgrimage to Jerusalem and died at Nicaea between 1-3 of July, 1035. His son William, aged about eight, succeeded him.