Buchanania onchidioides is a rare species of sea snail, a marine gastropod mollusk in the family Fissurellidae, the keyhole limpets and slit limpets.
The shell has an elongate-ovate shape, with a large central tubercle and radiating striae on a smooth mantle. The shell grows to a size of 80 mm. The oral appendages are simple, subulate and retractile.
This marine species occurs off Argentina and Chile.