thumb|Roger the Poitevin depicted in stained glass in [[Lancaster Priory]]
Roger the Poitevin or Roger de Poitou (mid-1060s – before 1140) was an Anglo-Norman aristocrat possessing large holdings both in England and through his marriage in France during the early 12th century.
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 divided between 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.
Roger's lordship extends beyond the Ribble as far as Cumberland
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 northern 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 1092, where Dolfin of Carlisle ruled, possibly as a vassal of Scottish King Malcolm Canmore. Dolfin was driven out and the Anglo-Scottish border was established north of Carlisle. Roger also acquired the great honour of Eye centered in Suffolk.
1088 and after
In 1088, he led a military force with Alan Rufus and Odo of Champagne against William de St-Calais, Bishop of Durham, at the request of William Rufus when the bishop was implicated in a revolt against the king; Roger also negotiated with the bishop on the king's behalf before the bishop went to trial.
Roger then went to his wife's holdings in Poitou. Almodis's uncle Odo was ousted as count of La Marche in 1104, and subsequently the sons of Roger and Almodis are styled as count. In 1109, Roger was permitted to briefly return to England to the court of Henry I,