The cap is fan-shaped to semicircular and convex, reaching a diameter of 2.5–12 cm (1.0–4.7 in).
It lacks a stalk, and is instead attached broadly to the substrate.
The fruit bodies are initially soft and spongy, but become hard and brittle once they have lost their moisture.
The cap surface is covered with matted hairs, has shallow concentric furrows, and a dull brownish-orange color.
The flesh is up to 3 cm (1.2 in) thick at the thickest part, and is a pale cinnamon color.
The pore surface is yellowish to brownish, and the pores are angular, numbering about 2–4 per millimeter.
In young fruit bodies, the pore surface bruises reddish brown.
The mushroom's odor and taste ranges from sweetish to indistinct.

Fruit bodies produce a white spore print.
Spores are elliptical to cylindrical, smooth, hyaline (translucent), and measure 3.5–5 by 2–3 μm.
The basidia (spore-bearing cells) are club shaped and four-spored, with dimensions of 18–22 by 5–5.6 μm.
H. nidulans has a monomitic hyphal system, containing only generative hyphae with clamp connections.
In the flesh, these hyphae are thick-walled and highly branched, measuring up to 10 μm in diameter; hyphae comprising the pores and the subhymenium are thinner (up to 6 μm wide) and less branched.
A chemical test can be used to help identify H. nidulans: all parts of the fruit body will instantly stain bright violet if a drop of an alkaline solution is applied.
Dilute (3–10%) potassium hydroxide (KOH), is often used for this purpose.
The cap is fan-shaped to semicircular and convex, reaching a diameter of 2.5–12 cm (1.0–4.7 in). It lacks a stalk, and is instead attached broadly to the substrate. The fruit bodies are initially soft and spongy, but become hard and brittle once they have lost their moisture. The cap surface is covered with matted hairs, has shallow concentric furrows, and a dull brownish-orange color. The flesh is up to 3 cm (1.2 in) thick at the thickest part, and is a pale cinnamon color. The pore surface is yellowish to brownish, and the pores are angular, numbering about 2–4 per millimeter. In young fruit bodies, the pore surface bruises reddish brown. The mushroom's odor and taste ranges from sweetish to indistinct.
Fruit bodies produce a white spore print. Spores are elliptical to cylindrical, smooth, hyaline (translucent), and measure 3.5–5 by 2–3 μm. The basidia (spore-bearing cells) are club shaped and four-spored, with dimensions of 18–22 by 5–5.6 μm. H. nidulans has a monomitic hyphal system, containing only generative hyphae with clamp connections. In the flesh, these hyphae are thick-walled and highly branched, measuring up to 10 μm in diameter; hyphae comprising the pores and the subhymenium are thinner (up to 6 μm wide) and less branched. A chemical test can be used to help identify H. nidulans: all parts of the fruit body will instantly stain bright violet if a drop of an alkaline solution is applied. Dilute (3–10%) potassium hydroxide (KOH), is often used for this purpose.
Hapalopilus nidulans