The House of Lancaster was a cadet branch of the royal House of Plantagenet. It was one of the opposing factions involved in the Wars of the Roses, an intermittent civil war which affected England and Wales during the 15th century. The family provided England with three kings: Henry IV of England, who ruled 1399–1413; Henry V of England, who ruled 1413–1422; and Henry VI of England and (II of) France, who ruled 1422–1461 and 1470–1471. The term "Lancastrian" refers to members of the family as well as their supporters.

The House descended from Edward III's third surviving son, John of Gaunt. Gaunt did not receive a large inheritance, so he made his fortune through marriage to the heiress Blanche of Lancaster, who brought with her the considerable lands of the Earls of Leicester and Lancaster, which made Gaunt the wealthiest landowner in England after the King. He was conferred the second creation of the title of "Duke of Lancaster" by his father Edward III in 1362, a year following the death of John of Gaunt's father-in-law Henry of Grosmont, 1st Duke of Lancaster. Gaunt enjoyed great political influence during his lifetime, but upon his death in 1399, his lands were confiscated by Richard II.