Uther Pendragon in the context of "Ambrosius Aurelianus"

The size of King Arthur's family mirrored the size of his legend. Although always large, it particularly grew as the legend of King Arthur gained popularity throughout Britain. According to the earliest Welsh Arthurian tradition, Arthur has an extensive family network. This includes his parents Uther Pendragon and Eigyr (Igraine), his wife Gwenhwyfar (Guinevere), his nephew Gwalchmei (Gawain), a brother, and several sons. His maternal lineage is also detailed, which includes his grandfather Amlawdd Wledig, a legendary king. His family tree was both simplified and expanded in shared British and French traditions. The two countries added characters from other medieval European chronicles and romances, which introduced new characters, such as Arthur's half-sisters, including Morgan, and their children, including Mordred. Various rulers claimed to be descended from Arthur, in particular the House of Tudor and Scottish clans, reflecting the importance of his family legend in medieval and early modern genealogies.

King Lot /ˈlɒt/, also spelled Loth or Lott (Lleu or Llew in Welsh), is a British monarch in Arthurian legend. He was introduced in Geoffrey of Monmouth's pseudohistorical Historia Regum Britanniae (c. 1136) as King Arthur's brother-in-law, who serves as regent of Britain between the reigns of Uther Pendragon and Arthur. He has appeared regularly in works of chivalric romance, alternating between the roles of Arthur's enemy and ally, and is often depicted as the ruler of Lothian and either Norway or Orkney. His literary character is probably derived from hagiographical material concerning Saint Kentigern, which features Leudonus as king of Leudonia (the Latin name for Lothian) and father of Saint Teneu.

Lot is generally portrayed as the husband of Arthur's sister or half-sister known by many names but most often as Anna, Gwyar, or variants of Morgause. The number and names of their children vary depending on the source, but usually they prominently include Gawain as well as other sons and sometimes daughters. Geoffrey suggests Lot was also the biological father of Mordred. The later prose romances identify him as the father of Gawain's younger brothers Agravain, Gaheris, and Gareth, while turning Mordred into an incestuous son of Arthur.

Morgause (/ˈmɔːrɡoʊz, -ɡɑːz/ MOR-gohz, -⁠gahz) is a popular name of a legendary queen and member of King Arthur's family in the Matter of Britain literature, where she is always a queen and usually Arthur's sibling. However, her name varies between texts and traditions, including Anna, Gwyar, or simply as the Queen of Orkney, as does the issue of her children, other than commonly Gawain. In most cases, she is the wife or widow of Lot, ruling over a northern realm such as Orkney, Lothian, Scotland, or Norway. She often has sisters, notably Morgan, with whom she is being sometimes conflated into a single character by modern authors.

In medieval chronicles and Arthurian romances based on or inspired by Geoffrey of Monmouth's Historia Regum Britanniae, as well as in the Welsh tradition, she is typically depicted as the daughter of Igraine, either the daughter or step-daughter of Uther Pendragon, and the full sister or half-sister of Arthur. Occasionally, she may be Uther's sister and Arthur's aunt. Her other children often, but not always, include Mordred.