Arthonia thoriana was described as a new species by the lichenologists Damien Ertz and Nicholas Sanderson in 2018.

The type specimen was collected in Great Britain, specifically in Horner Combe, Somerset, from the bark of ancient oak trees.

The species epithet honours the Swedish lichenologist Göran Thor, "for his outstanding work on the taxonomy of the Arthoniaceae".

It is non-lichenised despite the presence of large colonies of free-living single-celled green algae.

[2] Arthonia thoriana is found on the dry bark of ancient Quercus petraea in grazed pasture woodlands in Horner Combe, Somerset, at elevations of 110–180 metres.