Cladonia grayi, commonly known as Gray's cup lichen or Gray's pixie cup, is a species of fruticose lichen in the family Cladoniaceae. It is characterised by small, leaf-like squamules forming its primary thallus and distinctive upright podetia (5–15 mm tall) that develop into goblet-shaped cups. The species contains several unique lichen substances, primarily grayanic acid, which causes it to glow light blue under ultraviolet light, and has also been found to produce protective pyrrolopyrazine compounds. Its photobiont is the green algal species Asterochloris glomerata.

The fungal component shows distinctive growth responses during development that occur only when encountering compatible algal cells, with the fungus growing extremely slowly at ≤1 cm per year in culture. Though found worldwide, C. grayi is most abundant in Arctic and temperate regions of the Northern Hemisphere. It typically grows on acidic substrates including rotting wood, organic soil layers, and bare ground, showing particular abundance in pine forests across a range of humidity conditions. Two chemical variants (chemotypes) have been documented: one containing only grayanic acid and 4-O-demethylgrayanic acid, and another that additionally contains substances of the fumarprotocetraric acid complex.

The type specimen of Cladonia grayi was collected in 1928 near Long Creek in Charlotte, Mecklenburg County, North Carolina (originally cited as "North Virginia"). It was distributed as part of Sandstede's Cladoniae Exsiccatae collection, number 1847. The lectotype specimen is housed at the Farlow Herbarium (FH) at Harvard University and was designated by Teuvo Ahti in 1993 in his work published in Regnum Vegetabile.

Several forms of this species have been proposed, but it is unclear if these historical infrataxa have independent taxonomic significance. As Evans noted about form simplex, "the present form may represent either a stage in the development of one of the more complex forms or a definitely arrested conditions which will not develop further."

Common names that have been used for Cladonia grayi include "Gray's cup lichen" and "Gray's pixie cup".

Cladonia grayi is characterised by two main parts: a primary thallus consisting of small, persistent leaf-like structures (squamules), and a secondary thallus comprising upright stalks called podetia. The squamules, which form a crusty layer on the substrate, are small (1 mm long and 1–2 mm wide) with wavy edges and tend to grow upward.

The podetia, which are the most distinctive feature of the species, grow 5–15 mm tall and 2–7 mm wide. They are shaped like goblets (scyphose) with long, narrow stalks expanding into round, globe-like cups at the top. These structures are typically light greyish-green but can appear dark or somewhat brownish. The surface of the podetia is covered with a protective layer (cortex) that can be bumpy and may peel off in patches as the lichen ages. The upper portions develop powdery areas (soralia) and granules measuring 40–130 micrometres in diameter, or they may produce small flaking squamules. The granules are thought to play a play in propagation of the species.

The species' reproductive structures include dark brown spore-producing discs (apothecia) that appear uncommonly along the cup margins, and more frequent smaller structures called pycnidia that produce a clear fluid.