Nephroma arcticum, the arctic kidney lichen, is a species of foliose (leafy), terricolous (ground-dwelling) lichen in the family Parmeliaceae. It has a yellowish-green thallus up to 8 cm (3 in) across made of large lobes, with a dark, tomentose lower surface. It is a tripartite lichen, consisting of a fungus and two photobiont partners: a species of nitrogen-fixing cyanobacteria (contained within dark, blistered cephalodia), and a species of green alga. First described by Carl Linnaeus in 1753, it is one of only two Nephroma species in North America that use green algae as their primary photobiont.

Nephroma arcticum is widely distributed throughout the circumpolar regions of Asia, Europe, and North America, where it grows in semi-open coniferous forests and mountainous birch forests on shaded and moist ground, on moss carpets or on mossy rocks. The species is ecologically significant for its role in nitrogen fixation and as a food source for various animals including reindeer and mountain goats. Alaska Natives have historically used it both as food and traditional medicine. While generally secure across its range, its abundance varies regionally, being common in many arctic and subarctic areas but rare or imperilled in some southern portions of its distribution.

The species has been the subject of extensive research into its photosynthetic characteristics, nitrogen metabolism, and bacterial associations. Studies have revealed several key adaptations of N. arcticum to arctic conditions, including its reflective thallus surface that regulates light exposure and physiological mechanisms that enable photosynthesis during short summer seasons. The species shows considerable variation in its symbiotic relationships, with high genetic diversity among its cyanobacterial partners and the ability to develop separate cyanobacterial growth forms under certain conditions. The lichen's southern distribution limits are determined primarily by biological factors, particularly gastropod grazing of nitrogen-rich cyanobacterial tissues, rather than by temperature tolerance.

The species, originally named Lichen arcticus, was one of the first few dozen lichen species formally described by the Swedish botanist Carl Linnaeus in his 1753 work Species Plantarum. Linnaeus derived the diagnostic phrase from his earlier Flora Svecica (1745), adding a single descriptive term, laevis (smooth). He also referenced Flora Lapponica (1737). Linnaeus's original concept of the species was represented by five herbarium sheets in the Linnaean Herbarium (LINN), all annotated by Linnaeus himself. Two of these specimens were later selected as possible lectotypes: Reginald Heber Howe, Jr. (1912) designated specimen 1273.183, while Peter James and F. Joy White (1987) designated specimen 1273.180. Howe's typification has priority, and specimen 1273.183, which bears numbers from both Species Plantarum and Flora Svecica, is regarded as the lectotype, serving as the generitype of Nephroma.

Linnaeus's brief diagnosis included several of its key features:

Linnaeus noted that the lichen grows on the ground, particularly under junipers in northern Sweden. He further described the shield (apothecia) underside as "large, livid-flesh-coloured, about the size of a thumb", remarking that it is among the widest of all known lichens at the time. Linnaeus's description summarised what he considered the distinctive features of this species that allowed for its identification and differentiation from other lichens in the region.

Since its original description, the taxon has been proposed for inclusion in several genera throughout its taxonomic history, including Peltigera (Raeusch., 1797), Peltidea (Göran Wahlenberg, 1812), Parmelia (Linds., 1871), Nephromium (Johan Havaas, 1897), and Opisteria (Edvard Vainio, 1909). The Swedish lichenologist Gustav Torssell transferred it to the genus Nephroma in 1843, establishing the binomial name by which it is now known.

A 2002 molecular phylogenetics study confirmed that Nephroma forms a monophyletic group with Peltigera as its sister genus. Within Nephroma, N. arcticum was found to be relatively divergent from other species, suggesting an early evolutionary split. The study also demonstrated that tripartite species like N. arcticum, which contain both green algal and cyanobacterial photobionts, do not form a monophyletic group within the genus, indicating that the transition between bipartite and tripartite forms has occurred multiple times during the evolution of Nephroma.