Leir was a legendary king of the Britons as accounted by Geoffrey of Monmouth. He was the son of King Bladud and a source for William Shakespeare's King Lear.

Leir followed his father Bladud to the kingship of Britain and had the longest reign of the all the kings at sixty years. He built the city of Kaerleir (Leicester) along the banks of the River Soar.

Unlike his predecessors, he produced no male heir to the throne but had three daughters: Goneril, Regan, and Cordelia, whom he favored the most. As he neared his death, he divided the kingdom between his three daughters and their husbands. Goneril and Regan flattered their father and were married off to the Duke of Cornwall and Duke of Albany respectively, each being promised half of the kingdom to inherit. Cordelia, however, refused to flatter her father and was given no land to rule. Aganippus, the king of the Franks, courted Cordelia and married her, although Leir refused any dowry with her marriage.

Some time after this, Leir became old and the two dukes whom had married his daughters rebelled and seized the whole of the kingdom. Maglaurus, the Duke of Albany, maintained Leir in his old age, protecting him with 140 knights. Goneril, however, disapproved of this and after two years, she decreased Leir's bodyguard to only thirty. He fled to Cornwall and Regan decreased his guard to only five knights. He fled back to Albany and pleaded with Goneril, but he was awarded with only one knight for protection.

Fearing his two daughters, he fled to Gaul and his youngest daughter, Cordelia. Nearing insanity, he went to his daughter and she nursed him back to health. He was held in high honour in Gaul by the leaders and they vowed to restore him to his former glory. Leir, Cordelia, and Aganippus invaded Britain at the head of a large army and overthrew the dukes and their wives. He reclaimed the throne of Britain and reigned for three more years until the time of his death. He was succeeded by his daughter, Cordelia.

She buried him in an underground chamber beneith the River Soar near Leicester. It was dedicated to the Roman god Janus and every year people celebrated his feast-day near Leir's tomb.