Sir Lucan the Butler is a servant of King Arthur and one of the Knights of the Round Table in the Arthurian legend. The duties of a "butler" have changed over time; Lucan was supposed to have been in charge of the royal court, along with Bedivere the Marshal and Kay the Seneschal.

Lucan is the son of Duke Corneus, brother to Sir Bedivere and cousin to Sir Griflet. He and his relatives are among Arthur's earliest allies in the fight against the rebel kings such as Lot, Urien, and Caradoc, and remained one of Arthur's loyal companions throughout his life. In most accounts of Arthur's death, from the Lancelot-Grail Cycle to Malory's Le Morte d’Arthur, Lucan is one of the last knights at the King's side at the Battle of Camlann. He is usually the last to die; he helps Arthur off the battlefield after he battles Mordred, but the stress is too much. He dies from his own wounds just before the King returns Excalibur to the Lady of the Lake and sails off for Avalon.

Though the knight Arthur asks to cast the sword into the lake is usually Griflet (Lancelot-Grail) or Bedivere (Le Morte d'Arthur, the Alliterative Morte Arthure, the Stanzaic Morte Arthur), the 16th century English ballad "King Arthur's Death" ascribes this duty to Lucan.

He was a solid and reliable Knight of the Round Table and one of King Arthur's earliest companions. He took on the post of Royal butler - an important position in charge of the Royal Household rather than a serving man. He valiantly defended Arthur's right to the throne at the Battle of Bedegraine and probably against subsequent rebellions. Though he sought adventure, he never came to the fore in Arthurian tales with renowned exploits of his own. He always attended the Royal tournaments and was once hurt so badly by Sir Tristan that Sir Yvain had to escort him to Gannes Abbey for medical assistance. Sir Lucan remained loyal to King Arthur throughout the schism with Lancelot and on occasion acted as their go-between. Similarly during Mordred's rebellion he stayed by the monarch's side and though wounded, with his brother, Bedivere, he was one of the few knights left standing at the Battle of Camlann. He tried to dissuade Arthur from his final attack on his son/nephew, but was unsuccessful and the King received his mortal wound. Worried about looters on the battlefield, Lucan and Bedivere attempted to move the dying Arthur into a nearby chapel for safety; but the strain was too much for Lucan. A severe wound burst open, spilling out his bowels, and he died.

A character named Lucan appears in the 2004 film King Arthur, played by Johnny Brennan. He is a young boy found and cared for by Arthur's knight Dagonet.

Sir Lucan was a son of Duke Corneus and probably maternal half-brother of Sir Bedivere.