He is married to Margaret Beaufort.
They got married on April 23, 1398, he was 25 years old.Source 2
Child(ren):
John Beaufort, 1st Marquess of Somerset and 1st Marquess of Dorset, later only 1st Earl of Somerset, KG (1373 \endash 16 March 1410) was the first of the four illegitimate children of John of Gaunt, 1st Duke of Lancaster, and his mistress Katherine Swynford, later his wife. Beaufort was born in about 1371 and his surname probably reflects his father's lordship of Beaufort in Champagne, France.[1]
The family emblem was the portcullis which is shown on the reverse of a modern British 1p coin. John of Gaunt had his nephew Richard II declare the Beaufort children legitimate in 1390,[2] John of Gaunt married Katherine Swynford in January 1396. Despite being the grandchildren of Edward III, and next in the line of succession after their father's legitimate children, the Lancasters, the Beauforts, including John Beaufort, were barred from succession to the throne.