King Lear is a tragedy written by William Shakespeare. It depicts the gradual descent into madness of the title character, after he disposes of his kingdom giving bequests to two of his three daughters based on their flattery of him, bringing tragic consequences for all. Derived from the legend of Leir of Britain, a mythological pre-Roman Celtic king, the play has been widely adapted for the stage and motion pictures, with the title role coveted by many of the world's most accomplished actors.

The first attribution to Shakespeare of this play, originally drafted in 1605 or 1606 at the latest with its first known performance on St. Stephen's Day in 1606, was a 1608 publication in a quarto of uncertain provenance; it may be an early draft or simply reflect the first performance text. The Tragedy of King Lear, a more theatrical revision, was included in the 1623 First Folio. Modern editors usually conflate the two, though some insist that each version has its own individual integrity that should be preserved.

After the English Restoration, the play was often revised with a happy, non-tragic ending for audiences who disliked its dark and depressing tone, but since the 19th century Shakespeare's original version has been regarded as one of his supreme achievements. The tragedy is particularly noted for its probing observations on the nature of human suffering and kinship. George Bernard Shaw wrote, "No man will ever write a better tragedy than Lear."