King Leir is an anonymous Elizabethan play about the life of the ancient Brythonic king Leir of Britain. It was published in 1605 but was entered into the Stationers' Register on 15 May 1594. The play has attracted critical attention principally for its relationship with King Lear, Shakespeare's version of the same story.

Leir was a legendary king of the Britons whose story was recounted by Geoffrey of Monmouth in his pseudohistorical 12th-century History of the Kings of Britain. According to Geoffrey's genealogy of the British dynasty, Leir's reign would have occurred around the 8th century BC, around the time of the founding of Rome. The story was modified and retold by William Shakespeare in his Jacobean tragedy King Lear.