Roger the Poitevin (Roger de Poitou) was born in Normandy in the mid-1060s and died before 1140. He was an Anglo-Norman aristocrat, possessing large holdings in both England and through his marriage in France.
He was the third son of Roger of Montgomery, 1st Earl of Shrewsbury and Mabel de Bellême. The appellation "the Poitevin" was for his marriage to an heiress from Poitou.
Roger acquired a great lordship in England, with lands in Salfordshire, Essex, Suffolk, Nottinghamshire, Derbyshire, Lincolnshire, Hampshire and North Yorkshire. The principal part of the Lordship was in what was then called inter Mersam et Ripam, that is, "between the Mersey and the Ribble" and is now part of divided into Lancashire, Merseyside, and Greater Manchester. After 1090, he also assumed the title 1st Lord of Bowland.
Before 1086, he had married Almodis, daughter of Count Aldebert II of La Marche in Poitou, and sister and presumptive heiress of count Boso III who was childless and unmarried.
Around 1091, Roger's brother-in-law Boso died, but Roger was apparently preoccupied with Norman and English affairs, and his wife's uncle Odo became count of La Marche.
In 1092 Roger acquired a large part of what is now north Lancashire. These grants gave Roger effective control of all the lands north of the River Ribble to the River Lune, which formed a natural border between the secure Norman lands in England and the strongly contested Scottish frontier lands in Cumberland. Due to long established lines of communication across Morecambe Bay, Roger also assumed authority over the regions of Furness and Cartmel; these remained a part of Lancashire until as recently as 1974. The expansion of Roger's lands followed his support of King William II Rufus's invasion of Cumbria in AD1092, where Dolfin of Dunbar probably ruled as a vassal of Scottish King Malcolm Canmore. Dolfin was driven out and the Anglo-Scottish border was established north of Carlisle.