Hypogymnia congesta has a brown to brownish-grey foliose thallus measuring up to 8 cm (3.1 in) long or broad, with a cartilage-like texture.

[3] The thallus of Hypogymnia congest is appressed to somewhat erect and can reach up to 8 cm (3.1 in) in width or length.

The upper surface is smooth to weakly rugose and varies in colour from brown to brownish-grey.

Virensic acid produces pale violet-gray spots in thin-layer chromatography, with specific retardation factor (Rf) values that help identify this unique chemical constituent.

The cortex and medulla exhibit various colour reactions when tested with different reagents, providing another means to differentiate this species from others.