Robert I of Flanders, known as Robert the Frisian, was count of Flanders from 1070 to 1092.

He was the younger son of Baldwin V of Flanders and Adela (Alix) Capet, a daughter of King Robert II of France.

Robert was originally intended to secure the northern borders of Flanders by his marriage to Gertrude of Holland. But after his brother's death in 1070 he displaced his nephews and became count of Flanders.

By Gertrude of Holland he had 5 children:

• Robert II of Flanders
• Adela (d.1115), who first married king Canute IV of Denmark, and was the mother of Charles the Good, later count of Flanders. She then married Roger Borsa, duke of Apulia.
• Gertrude, who married Thierry II, Duke of Lorraine, and was the mother of Thierry of Alsace, also later count of Flanders
• Philip of Loo, whose illegitimate son William of Ypres was also a claimant to the county of Flanders
• Ogiva, abbess of Messines