John Beaufort, 1st Earl of Somerset (c. 1373 - March 16, 1410) was the first of the four children of John of Gaunt, Duke of Lancaster, and his mistress (later his wife) Katherine Swynford. Beaufort was born in about 1373 and his surname probably reflects his birthplace, his father's Beaufort Castle in Anjou, France. The family emblem was the portcullis. John of Gaunt had his nephew Richard II of England declare the Beaufort children legitimate in 1390, with the important proviso that they were barred from the succession to the throne, despite being the grandchildren of Edward III of England. It is thought that this may have been a "private" act (that is, not entered in the public records), because, in January 1397, the Duke had Parliament issue a similar declaration, with the same proviso. Later that month, Gaunt married Katherine although they had been living apart for some years, possibly in order to have their children publicly declared legitimate. While this legal wrangling ultimately caused an enormous amount of bloodshed and destruction, it did result in one of the Beaufort descendants ascending the throne as Henry VII.

John Beaufort was created Earl of Somerset on February 10, 1397. On September 28 of that year he married Margaret Holland, daughter of Thomas Holland, 2nd Earl of Kent; the next day he was created Marquess of Somerset and Dorset. Also sometime that year he was made a Knight of the Garter. He lost the two marquessates in 1399 after the accession of Henry IV. In 1404 he was Constable of England.

He and his wife had six children:

• Henry Beaufort, 2nd Earl of Somerset (c. 1401 - November 25, 1418).
• John Beaufort, 1st Duke of Somerset (baptized March 25, 1404 - May 27, 1444).
• Thomas Beaufort, Earl of Perche (c. 1405 - 1432).
• Joan Beaufort (c. 1406 - July 15, 1445), who married James I of Scotland.
• Edmund Beaufort, 2nd Duke of Somerset (c. 1406 - May 22, 1455).
• Margaret Beaufort (c. 1409 - 1449).