Robert I (c. 1000 – July, 1035), called "The Magnificent" (French, "le Magnifique") for his love of finery, was Duke of Normandy from 1027 to 1035. He was the son of Duke Richard II of Normandy and Judith, daughter of Conan I, Duke of Brittany.

When his father died, he became count of Hiémois . He succeeded his elder brother, Richard III of Normandy, a year later when Richard died. There were suspicions that Robert had Richard murdered, hence his nickname, "Robert le diable" (the devil). He is often identified with the legendary Robert the Devil. He 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, then in exile at Robert's court, and sponsored monastic reform in Normandy.

By his mistress, Herleva, he was father of the future William the Conqueror. An illegitimate daughter, Adelaide, became Countess of Ponthieu and Countess of Champagne.

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.