[2] It was formally described as a new species in 1805 by the Swiss botanist Augustin Pyramus de Candolle, who originally classified it in the genus Patellaria.

Porpidia macrocarpa is a crustose lichen with a variable thallus, typically appearing immersed (embedded in the substrate) to thin and continuous, but it can sometimes be thicker and become cracked (rimose) or divided into small sections (areolate).

They are constricted at the base with a thick, swollen true exciple (rim) that is persistent, black, shiny, and raised, ranging from entire to somewhat wavy (flexuose).

The disc of the apothecia can be slightly concave to flat or convex, black or brown-black, with a matt or shiny finish, and sometimes covered with a grey powdery coating (pruina).

[4] Lichenicolous (lichen-dwelling) fungi that have been recorded growing on Porpidia macrocarpa include Cecidonia xenophana, Endococcus propinquus, E. rugulosus, Muellerella pygmaea, and Sclerococcum australe.