Porpidia pachythallina is a species of rock-dwelling, crustose lichen-forming fungus in the family Lecideaceae. Found in Great Britain, it was described as a new species in 2005 by Alan Fryday. The type specimen (holotype) was collected in Scotland (United Kingdom), in Mid Perthshire (vice-county 88) within Ben Lawers National Nature Reserve, at Burn of Edramucky. It was found growing on an acidic mica-schist boulder.