Slugs in the genus "Arion" have the pneumostome (respiratory pore) in the front part of the mantle and a round back without a keel. "Arion rufus" is one of the largest species in the genus, up to 150 mm extended. As an adult it lacks the lateral bands found in many smaller congeners, but in some forms the sides are paler than the back. The body colour is often reddish, sometimes vividly so, but orange, yellowish, brown and greyish forms are also usual, and some individuals may be black. The foot fringe is vertically striped and is often a different colour than the back. The head and tentacles are often darker than the rest of the body. The mucus is sticky and colourless or orange. Externally the species is not reliably distinguishable from several other species, including "A. ater" and "A. vulgaris", and can be difficult to distinguish from less closely related species such as "A. flagellus", especially in the Iberian Peninsula where further similar species exist.