Physcia caesia, known colloquially as blue-gray rosette lichen and powder-back lichen, is a species of foliose lichenized fungus. First described by Georg Franz Hoffmann in 1784, it is common across much of Europe, North America and New Zealand, and more patchily distributed in South America, Asia, Australia and Antarctica. There are 2 subspecies: P. c. caesia and P. c. ventosa, as well as a number of distinct forms and varieties. Molecular studies suggest that the species as currently defined may be polyphyletic. It is typically pale gray shading to darker gray in the center (though some forms are considerably darker), and grows in a small rosette, usually some 2–3 cm (0.79–1.18 in) across at maturity. It only rarely has apothecia, instead reproducing most often vegetatively via soredia, which are piled in round blue-gray mounds across the thallus's upper surface. It grows most often on rock—principally calcareous, but also basaltic and siliceous—and also occurs on bone, bark and soil. It is nitrophilic and is particularly common on substrates where birds perch.

Capable of growing at a multitude of angles on a variety of surfaces, Physcia caesia also tolerates a wide range of environmental extremes from the high temperatures of desert locations to the low temperatures of the Antarctic. It grows on both dry stone and that moistened by seepage, and can survive being irregularly submerged for extended periods underwater. Like many lichens that grow on rock, Physcia caesia is able to extract nutrients from the substrate upon which it grows, as well as from rainwater and atmospheric dust. It is threatened by habitat loss through development, as well as trampling or overgrowth of its location. A number of lichenicolous species are known parasites.

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