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.

In Welsh Arthurian pre-Galfridian tradition, Welsh sources laid out a few close familial figures; Arthur had a father named Uther Pendragon, a brother called Madog, and a nephew (Eliwlod). Arthur also appears to have had a sister in this tradition. She is unknown except in reference to Gwalchmei, son of Gwyar, who is said to be the child of Arthur's cousin and sister in Culhwch and Olwen. The Vita Iltuti and the Brut Dingestow both say that Arthur's mother was named Eigyr. Culhwch and Olwen also names a half-brother, Gormant, the son of Arthur's mother and Ricca, the chief elder of Cornwall. This parallels later stories of Gorlois, Duke of Cornwall.

The 13th-century genealogies in Mostyn MS. 117 lay out Arthur's ancestry in detail. He is the son of Uthyr, the son of Custennin, the son of Cynfawr, the son of Tudwal, the son of Morfawr, the son of Eudaf, the son of Cadwr, the son of Cynan, the son of Caradoc, the son of Bran, and the son of Llŷr. Regarding Arthur's own family, his wife is consistently said to be Gwenhwyfar. She is usually stated to have a sister named Gwenhwyfach and to be the daughter of King Ogrfan Gawr (who is sometimes called 'Gogrfan Gawr' or '[G]Ogrfan the Giant'). Culhwch and Bonedd yr Arwyr also indicate that Arthur had some sort of relationship with Eleirch, daughter of Iaen, resulting in a son named Kyduan (Cydfan). Kyduan is not Arthur's only child in Welsh Arthurian tradition – he is also said to have sons named Amr (Amhar), Gwydre, Llacheu and Duran. (See the Offspring section for further information about Arthur's children.)

In addition to this immediate family, Arthur was said to have had a great variety of distant relatives, including maternal aunts, uncles, and cousins, as well as a grandfather named Anlawd (or Amlawdd) Wledig ("Prince Anlawd"). Anlawd is the common link between Arthur and many of these figures. For example, the relationship between first cousins (implied or stated), between Arthur, Culhwch, Illtud, and Goreu fab Custennin, depends on all of their mothers being children of Anlawd. Arthur's maternal uncles in Culhwch and Olwen, including Llygatrud Emys, Gwrbothu Hen, Gweir Gwrhyt Ennwir, and Gweir Baladir Hir, are similarly related through Anlawd. Some argue that Anlawd only exists as a means to allow medieval Welsh authors to interconnect figures with Arthur by acting as a genealogical link.

Geoffrey of Monmouth carried over relatively few members of Arthur's family in the Welsh materials. Arthur's grandfather, Anlawd Wledic, and his maternal uncles, aunts, and cousins do not appear there, nor do his paternal relatives or any of his sons. Only the core family seems to have made the transition in Geoffrey's influential version: Arthur's wife Gwenhwyfar (who became Guinevere), his father Uthyr (Uther), his mother Eigyr (Igerna), and his nephew Gwalchmei (Gawain). Uther was given a new family, including two brothers and their father. Gwalchmei's mother, Gwyar, instead became Anna, who was married to Loth, while Modredus (Mordred) became her second son (he was not her son under Welsh tradition, bearing the name Medraut). As many writers based their stories on Geoffrey's work, it was his version that remained popular, rather than traditional Welsh family trees.

In the chivalric romance branch of such common tradition, Arthur gains a sister or half-sister named Morgan, first identified as his relative by Chrétien de Troyes' Yvain. Arthur's other sister or half-sister, today best known as Morgause, is a daughter of Gorlois and Igerna (Igraine). She replaced Anna in the romances as the mother of Gawain and Mordred. She and Morgan may also be joined by a third half-sister, today best known as Elaine. Drawing on earlier sources, Richard Carew mentions another sister of Igraine and Uther, named Amy. The overall number of Arthur's sisters or half-sisters varies between the different romances, ranging from as few as one or two to as many as five (in which case one of them may die early). Their names and roles also vary, as do their husbands (most commonly the British kings Lot, Urien, and Nentres, who are largely interchangeable). Through his sisters, Arthur is given further nephews, who all become members of the Round Table. The sisters usually (but not always) have particular children in the romances. In popular tradition of the prose cycles, Morgause has Gawain, Agravain, Gaheris, and Gareth; Elaine has Galeschin; and either Morgan or a fourth sister has Yvain. Other romance authors such as Chrétien and Wolfram von Eschenbach mention or feature Arthur's nieces and occasionally additional nephews (for example, Lancelot is the son of Arthur's unnamed sister in Ulrich von Zatzikhoven's Lanzelet, but nowhere else).

Arthur's son, named Loholt, was introduced in Chrétien de Troyes's Erec and Enide. He is possibly based on one of Arthur's sons from Welsh tradition, Llacheu. Geoffrey turns Ambrosius Aurelianus, the historical Romano-British leader, into Uther's brother. He also derives Arthur's lineage from the self-proclaimed Western Roman Emperor Constantine III of Britain, presenting him as Arthur's grandfather. Additionally, the chronicle Brut Tysilio makes Cador, son of Gorlois Arthur's half-brother, sharing Igraine as a mother. In Geoffrey's Historia, Cador's son Constantine succeeds Arthur as the high king of Britain. One important figure with no actual blood relation to Arthur is Ector, who is featured as a secret foster-father of Arthur in much of the romance tradition, along with Ector's son Kay as the young Arthur's foster-brother.

Although Arthur is given sons in both early and late Arthurian tales, he is rarely has many generations of offspring. This is at least partly because of the usually premature deaths of Arthur's sons. In some cases, including in Le Morte d'Arthur, Arthur's failure to produce a legitimate heir contributes to his fall.