Katherine Swynford (also spelled Synford), née (de) Roet (also spelled (de) Rouet or (de) Roelt; c. 1350 – May 10 1403), was the daughter of Payne (or Paen) de Roet, a Flemish herald from Hainault who was knighted just before his battlefield death. His children included Katherine, her older sister Philippa, a son, Walter, and the eldest sister, Isabel de Roet, (who died Canoness of the convent of St. Waudru's, Mons, c. 1366).

About the year 1366, at the age of 16, Katherine married Hugh Swynford (1340-1372), an English knight from the manor of Kettlethorpe in Lincolnshire, and bore him at least two children; Thomas (1368-1432), Blanche (born 1370), and likely the Margaret Swynford (born c. 1369) who was nominated a nun at the prestigious Barking Abbey by the command of Richard II in 1377). Katherine then became attached to the household of John of Gaunt, Duke of Lancaster, ostensibly as governess to his two daughters (the sisters of the future Henry IV of England) by his first wife Blanche. Eventually, she became his official mistress, about 1373. Katherine's sister Philippa, a member of the household of Queen Philippa of Hainault, wife of Edward III, married the poet Geoffrey Chaucer, whose poem The Book of the Duchess commemorated Blanche's death in 1369.

Long after the death of his second wife Constance of Castile, John and Katherine married on 13 January 1396 in Lincoln Cathedral, three years before he died. The four children Katherine had borne John of Gaunt had been given the surname "Beaufort" and were already adults when they were legitimized by this marriage with papal approval. The Beauforts were later barred from inheriting the throne by a clause inserted into the legitimation act by their half-brother, Henry IV.

Katherine survived John by only four years, dying on May 10, 1403. (Since she was then dowager Duchess of Lancaster, there was a record of the exact day, as there was not for her birth, when she was of considerably lower rank). Her tomb, and that of her daughter Joan Beaufort, are under a carved-stone canopy in the sanctuary of Lincoln Cathedral, but their remains are no longer in them, because the tombs were despoiled in 1644, during the English Civil War, by the Roundheads.

Katherine's children by John of Gaunt were:

*John Beaufort, 1st Earl of Somerset. *Henry Cardinal Beaufort. *Thomas Beaufort, 1st Duke of Exeter. *Joan Beaufort, Countess of Westmorland.

Katherine's son John was the great-grandfather of Henry VII of England and the grandfather of James II of Scotland; her daughter Joan Beaufort was the grandmother of Edward IV of England and Richard III of England, whom Henry VII defeated to take the throne. (Henry then married Elizabeth of York, daughter of Edward IV, and their son became Henry VIII of England). Her step-son became Henry IV of England by deposing Richard II of England (who was imprisoned and died shortly thereafter, in Pontefract Castle, where Katherine's son Thomas Swynford was constable, and he was said to have starved Richard to death for his step-brother); her step-daughter, John and Constance's daughter Catherine (or Catalina), was the great-grandmother of Catherine of Aragon, the first wife of Henry VIII of England and mother of Mary I of England.

Katherine Swynford is the subject of Anya Seton's novel Katherine (published in 1954) and of Alison Weir's biography Katherine Swynford: The Story of John of Gaunt and his Scandalous Duchess (ISBN 0224063219). Swynford is also the subject of Jeanette Lucraft's historical biography Katherine Swynford: The History of a Medieval Mistress. This book seeks to establish Swynford as a powerful figure in the politics of fourteenth-century England, and an example of how a woman could manipulate the social mores of the time for her own interests rather than just as the sexual temptress that previous writers have portrayed.

*The Katherine Swynford Society