Microcondylaea compressa can reach a length of about and a width of . Shells are quite elongated and laterally compressed. The external surface is light brown to brown with living mussels (empty shells are often coloured dark brown to black) and shows shallow concentric striae of growth. Typical for this species are the tree-like ("arboriform") siphonal papillae - all other European freshwater mussels have undivided papillae. This allows identification of the species without disturbing by touching it even when dug in completely.
