Pholiota squarrosa, commonly known as the shaggy scalycap, the shaggy Pholiota, or the scaly Pholiota, is a species of mushroom in the family Strophariaceae. Both the cap and the stem are covered in small, pointed scales that are pointed downward and backward. The crowded gills are yellowish, then later rust-brown. The mushroom has an odor that, depending on the author, has been described as resembling garlic, lemon, radish, onion, or skunk. It has a strong taste, resembling radishes. The very similar P. squarrosoides differs in having a paler cap that is sticky between the scales, and smaller spores.

Common in North America and Europe, it is a secondary parasite, in that it attacks trees that have already been weakened from prior injury or infection by bacteria or other fungi. It has a wide range of hosts among deciduous trees, although it can also infect conifers. It can also live as a saprobe, deriving nutrients from decomposing wood. Though edible to some, it may be toxic, especially if consumed in combination with alcohol. The mushroom contains unique chemicals thought to help it infect plants by neutralizing defensive responses employed by them.

The species was first described scientifically as Agaricus squarrosus in 1790 by Georg Christian Oeder, and later sanctioned under this name by Elias Magnus Fries in his 1821 Systema Mycologicum. It was transferred to the genus Pholiota by the German Paul Kummer. It is the type species of the genus Pholiota.

The specific epithet squarrosa is derived from Latin, and means "scurfy". The mushroom is commonly known as the "scaly Pholiota", the "shaggy scalycap", or the "shaggy Pholiota".

Like other Pholiota mushrooms, P. squarrosa has a scaly cap and stem. The cap ranges from 3 to 12 cm (1.2 to 4.7 in) in diameter, and depending on its age, can range in shape from bell-shaped to rounded to somewhat flattened. The cap color is yellowish-brown to tawny in older specimens. The scales on the cap are yellowish to tawny, and recurved.

The stem is 4 to 12 cm (1.6 to 4.7 in) long by 0.5 to 1.5 cm (0.20 to 0.59 in) thick, and roughly equal in width throughout. The partial veil that covers the young gills forms a thick, woolly ring on the upper part of the stem. Above the level of the ring, the stem is bare, while below it is scaly like the cap. The gills are covered by a partial veil when young and have a greenish-brown color; mature gills are rusty brown. They are crowded closely together, attached to the stem (adnate), and usually notched (sinuate).

The spore print is cinnamon or rusty brown. The spores are elliptic, smooth-walled, nonamyloid (not absorbing iodine when stained with Melzer's reagent), and measure 6.6–8 by 3.7–4.4 μm. The basidia (spore-bearing cells) are club-shaped, and four-spored, with dimensions of 16–25 by 5–7 μm.

Fruit bodies have an odour described variously as resembling garlic, radish, lemon, onion, or skunk, and taste like radish.