Megalomyrmex acauna is a Neotropical species of ants in the subfamily Myrmicinae. Known from Brasil and Peru. It is similar to M. leoninus-group. Its head width (HW) is 1.45—1.63 mm and head length (HL) is 1.68—1.75 mm. Its scapus length (SL) is 2.13—2.28 mm.