Mark of Cornwall (Latin Marcus Cunomorus, Cornish Margh, Welsh Cynfawr) was a king of Kernyw (Cornwall) in the early 6th Century AD. According to legend, he was a cousin of King Arthur and father (or uncle) of Tristan; a gravestone found in Cornwall has confirmed that Tristan was at least a member of Mark's family. Mark was said by some to be Sir Tristan's uncle, either maternal or paternal is not known. Mark sent Tristan as his proxy to fetch his young bride, the Princess Isolde (Isoult, Essylt, Iseult) from Ireland. Tristan and Isolde fell in love, and, with the help, of a magic potion, proceeded to have one of the stormiest love affairs in medieval literature.

In the 1970s TV show Arthur of the Britons, Mark was played by Brian Blessed.