Four new species and one new forma are described in the genus Porpidia. Porpidia flavocruenta Fryday & Buschbom, a member of the P. macrocarpa group that has previously been overlooked for ‘P. flavocoerulescens’, is reported from Austria, the British Isles, Scandinavia, Iceland, and North America (Alaska); P. islandica Fryday, Knoph & Hertel is reported from Iceland and Scotland; and P. pachythallina Fryday and P. striata Fryday from the British Isles only. The sorediate morph of P. superba is described as P. superba f. sorediata Fryday, known from the British Isles, Sweden, Svalbard, and North America (Maine). Variation in P. macrocarpa is discussed and the new combination P. macrocarpa f. nigrocruenta (Anzi) Fryday made. Secondary metabolite production is discussed and the variation in the production of chemosyndromes considered to be more variable than previously reported. The position of several other taxa is discussed and P. herteliana and P. musiva are reduced to synonymy with P. cinereoatra, P. calcarea to synonymy with P. superba, and P. diversa to synonymy with P. contraponenda. However, P. grisea and P. lowiana are provisionally maintained as distinct species from P. tuberculosa and P. cinereoatra respectively, although P. grisea has not been correctly recorded from the British Isles. The typification of Spiloma tuberculosa Sm., the basionym of Porpidia tuberculosa, is discussed and a lectotype proposed; the new combination Porpidia rugosa (Taylor) Coppins & Fryday is made and shown to be the correct name for P. glaucophaea; and Porpidia flavicunda (Ach.) Gowan is used for the esorediate taxon usually known as Porpidia flavocoerulescens because this epithet is to be proposed for rejection as it is considered to be of confused usage. Porpidia hydrophila is shown to be a member of the P. albocaerulescens group. Porpidia lowiana, P. nadvornikiana, and P. thomsonii are recorded for the first time from the British Isles, and P. macrocarpa f. nigrocruenta confirmed as a British taxon. Notes and a key are provided for all the species of the genus that have been reported from the area.