John Beaufort, 1st Marquess of Somerset and 1st Marquess of Dorset, later only 1st Earl of Somerset, KG (c. 1373 – 16 March 1410) was an English nobleman and politician. He was the first of the four illegitimate children of John of Gaunt (1340–1399) (third surviving son of King Edward III) by his mistress Katherine Swynford, whom he later married in 1396.
The Beaufort children were declared legitimate twice by parliament, first during the reign of King Richard II, in 1397, which was confirmed by Henry IV, as well as by Pope Boniface IX in September 1396. Even though they were the grandchildren of Edward III and next in the line of succession after their father's legitimate children by his first two wives, the Beauforts were barred from succession to the throne by their half-brother Henry IV.

John was created Earl of Somerset on 10 February 1397, just a few days after the legitimation of the Beaufort children was recognised by Parliament. The same month, he was also appointed Admiral of the Irish fleet, as well as Constable of Dover Castle and Warden of the Cinque Ports. In May, his admiralty was extended to include the northern fleet. That summer, the new earl became one of the noblemen who helped Richard II free himself from the power of the Lords Appellant. As a reward, he was created Marquess of Somerset and Marquess of Dorset on 29 September, and sometime later that year he was made a Knight of the Garter and appointed Lieutenant of Aquitaine. In addition, two days before his elevation as a Marquess he married the king's niece, Margaret Holland, sister of Thomas Holland, 1st Duke of Surrey, another of the counter-appellants. John remained in the king's favour even after his older half-brother Henry Bolingbroke (later Henry IV) was banished from England in 1398.

Effigy John Beaufort 1st Earl of Somerset.
James William Edmund Doyle, CC0, via Wikimedia Commons