The fruiting bodies of Agaricus augustus are large and distinctive agarics. The cap shape is hemispherical during the so-called button stage, and then expands, becoming convex and finally flat, with a diameter of up to 22 cm. The cap cuticle is dry, and densely covered with concentrically arranged, brown-colored scales on a white to yellow background.
The flesh is thick, firm and white and may discolor yellow when bruised. The gills are crowded and pallid at first, and turn pink then dark brown with maturity. The gills are not attached to the stem — they are free. Immature specimens bear a delicate white partial veil with darker-colored warts, extending from the stem to the cap periphery.
The stem is clavate up to 20 cm tall, and 4 cm thick. In mature specimens, the partial veil is torn and left behind as a pendulous ring adorning the stem. Above the ring, the stem is white to yellow and smooth. Below, it is covered with numerous small scales. Its flesh is thick, white and sometimes has a narrow central hollow. The stem base extends deeply into the substrate.
The mushroom’s odor is strong and nutty, of anise or almonds, which can be associated with the presence of benzaldehyde and benzyl alcohol. Its taste has been described as not distinctive.
Under a microscope, the ellipsoid-shaped spores are seen characteristically large at 7–10 by 4.5–6.5 μm. The basidia are 4-spored. The spore mass is colored chocolate-brown.