Ornithoptera aesacus, the Obi Island birdwing, is an extremely rare species of birdwing butterfly, endemic to the Island of Obira (formerly Obi), Indonesia.

The specific epithet of Ornithoptera aesacus, is named after Æsacus, the eldest son of Priam.

The full reference is Ney, F. 1903 Eine neue Troides-Form von Obi.

They were once held by Hermann Rolle, whose collection, in part, was sold to Eugène Le Moult.

The two species are very similar but the male O. aesacus has a brilliant turquoise-blue sheen.