Acanthothecis megalospora is a species of corticolous (bark-dwelling) lichen in the family Graphidaceae.

Found in Brazil, it was formally described as a new species in 2022 by Shirley Cunha Feuerstein and Robert Lücking.

The type specimen was collected from a São Paulo farm near Itaguatins (Tocantins); here it was found growing on tree bark in cerrado.

Its asci contain a single, more or less rectangular ascospore (measuring 135–185 by 27–30 μm) with 15 to 17 transverse septa.

The specific epithet refers to these large spores.