| Geoffrey I | |
| Spouse: | Hawise |
| Issue: | Alan III, Duke of Brittany Odo, Count of Penthièvre |
| House: | House of Rennes |
| Father: | Conan I, Duke of Brittany |
| Mother: | Ermengarde-Gerberga of Anjou |
| Birth Date: | c. 980 |
| Birth Place: | Rennes, County of Rennes, Duchy of Brittany |
| Death Date: | 20 November 1008 |
| Death Place: | Nantes, Loire-Atlantique, Pays de la Loire, France |
| Burial Place: | Brittany |
Geoffrey I (c. 980 - 20 November 1008), also known as Geoffrey of Rennes and Geoffrey Berengar, was Duke of Brittany from 992 until his death, and also Count of Rennes (ruler of the Romano-Frankish civitas of Rennes) by right of succession. The eldest son of Duke Conan I of Brittany, he assumed the title of Duke of Brittany upon his father's death in 992. Brittany had long been an independent state, but he had little control over much of Lower Brittany.
Geoffrey was the son of Duke Conan I, by his marriage to Ermengarde-Gerberga of Anjou. He was the grandson of Judicael Berengar, Count of Rennes.
When Geoffrey succeeded to Brittany he had several problems:
In 996, at about the age of sixteen, Geoffrey entered into a dynastic alliance with Richard II, Duke of Normandy, with a diplomatic double marriage between the two houses. The church-sanctioned marriage ceremonies were held at Mont Saint-Michel, on the Breton-Norman border, and while Geoffrey married Hawise of Normandy, daughter of Richard I of Normandy and sister of Richard II. Richard married Judith of Brittany, Geoffrey's sister.
Geoffrey and Hawise had four children:
Geoffrey died on 20 November 1008 while travelling on a pilgrimage to Rome.