Ranulph I de Mortimer (Ralf, Ralph, Raoul de Mortemer) (fl. c.1080–1104) was a Marcher Lord from the Montgomery lands in the Welsh Marches (border lands between Wales and England). In England, he was Lord of Wigmore in  Herefordshire. In Normandy, he was the Seigneur of St. Victor-en-Caux. Ranulph was the founder of the English House of Mortimer of Wigmore. He acquired Wigmore Castle after William Fitz Osbern's son Roger de Breteuil joined the Revolt of the Earls of 1075. His lands and holdings in Herefordshire and Shropshire  were granted to him by William the Conqueror before 1086.