Katherine (or Katharine or Catherine) (c. 1350 – 1403) was the daughter of Payne (or Paen) de Roet (or Rouet or Roelt) a Flemish herald from Hainault who was knighted just before dying in the wars, leaving Katherine and her older sister Philippa, as well as a brother, Walter, and eldest sister, Isabel (Elizabeth) de Roet, (who died chanoinness of the convent of St. Waudru's, Mons, c. 1366). About the year 1366, at the age of 16, Katherine married Hugh Swynford or Synford, an English knight from the manor of Kettlethorpe in Lincolnshire, and bore him at least two children (Blanch, Thomas, and likely the Margaret Swynford who was nominated a nun at the prestigious Barking Abbey by the command of Richard II in 1377) before he, too, died in the European wars. She 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, but eventually she became his official mistress. Katherine's sister Philippa 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 (or Constanza) 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 (but barred from inheriting the throne by a clause inserted by half-brother Henry IV well into the latter's reign) in 1390:
• John Beaufort, 1st Earl of Somerset.
• Henry Cardinal Beaufort.
• Thomas Beaufort, 1st Duke of Exeter.
• Joan Beaufort, Countess of Westmorland.

Katherine survived John by only four years, dying on May 10, 1403. 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.