Knightley has starred in the "Pirates of the Caribbean" trilogy, as well as "Atonement," "Pride & Prejudice," "Love, Actually" and "Domino."
"Lear" is about an aging monarch who decides to divide his kingdom between his three daughters, based on how well they declare their love for him.
Knightley will play the title character's youngest, favored daughter, Cordelia. Anthony Hopkins will play Lear and Gwyneth Paltrow will play the role of Regan, Lear's treacherous middle daughter, The Daily Telegraph reported.
The role of Lear's eldest child, Goneril, has not yet been cast.
The $35 million film version of "Lear" was announced at the Cannes Film Festival in France, the British newspaper said.