Found in Sri Lanka, it was formally described as a new species in 2014 by lichenologists Gothamie Weerakoon, Robert Lücking, and Helge Thorsten Lumbsch.

The type specimen was collected from the Elkaduwa Plantation in Matale (Central Province) at an altitude of 915 m (3,002 ft).

The lichen is only known to occur at the type locality, which is a patch of mid-elevation, disturbed montane forest.

Rhabdodiscus isidiatus has a cream-colored to white, smooth to uneven thallus up to 5 cm (2 in) in diameter.

Its ascospores are hyaline, ellipsoid in shape, contain three thick septa, and measure 12–15 by 5–6 μm.