Adults are usually 20–35 mm long.
The skin and flesh are watery and fairly transparent.
The colour of the skin varies between light greyish-brown to almost black.
Close inspection reveals fine dark spotting usually over the whole body; this shows up better in alcohol-preserved specimens.
Often, but not always, the respiratory pore is pale and unspotted.
The mucus is colourless.

Particularly in North America, some Deroceras laeve grow large enough that they closely resemble D. invadens.
A useful clue to distinguish them is the profile of the end of the tail when the animal has been disturbed or is preserved.
The tail of D. invadens usually slants vertically upward from the sole for a short distance, or even bends backwards.
The tail of D. laeve slopes forward above the sole.
Also, the tail of D. invadens is longer than the mantle, whereas it is the same length or shorter in D. laeve.

Nevertheless, dissection is required to distinguish D. invadens reliably from D. laeve and from various similar Deroceras species occurring in Europe, such as Deroceras sturanyi, the true Deroceras panormitanum, and Deroceras golcheri.
Reise et al. (
2011) discuss and figure the most critical anatomical characters.
In most populations of D. invadens, the proximal penis has two side pockets (the penial caecum and penial lobe); these have roughly equal widths and both have rounded, stout ends.
A prominent appending penial gland with 3–7 branches attaches between the caecum and lobe on the dorsal side; these branches are less knobbly in outline than those of D. panormitanum.
The penial retractor muscle attaches between the caecum and lobe on the ventral side.
The intestinal caecum is either absent or represented merely by a widening of the rectum.
Barker (1999) and Sirgel (1973) detail various other aspects of the anatomy.
Adults are usually 20–35 mm long. The skin and flesh are watery and fairly transparent. The colour of the skin varies between light greyish-brown to almost black. Close inspection reveals fine dark spotting usually over the whole body; this shows up better in alcohol-preserved specimens. Often, but not always, the respiratory pore is pale and unspotted. The mucus is colourless.
Particularly in North America, some Deroceras laeve grow large enough that they closely resemble D. invadens. A useful clue to distinguish them is the profile of the end of the tail when the animal has been disturbed or is preserved. The tail of D. invadens usually slants vertically upward from the sole for a short distance, or even bends backwards. The tail of D. laeve slopes forward above the sole. Also, the tail of D. invadens is longer than the mantle, whereas it is the same length or shorter in D. laeve.
Nevertheless, dissection is required to distinguish D. invadens reliably from D. laeve and from various similar Deroceras species occurring in Europe, such as Deroceras sturanyi, the true Deroceras panormitanum, and Deroceras golcheri. Reise et al. (2011) discuss and figure the most critical anatomical characters. In most populations of D. invadens, the proximal penis has two side pockets (the penial caecum and penial lobe); these have roughly equal widths and both have rounded, stout ends. A prominent appending penial gland with 3–7 branches attaches between the caecum and lobe on the dorsal side; these branches are less knobbly in outline than those of D. panormitanum. The penial retractor muscle attaches between the caecum and lobe on the ventral side. The intestinal caecum is either absent or represented merely by a widening of the rectum. Barker (1999) and Sirgel (1973) detail various other aspects of the anatomy.
Deroceras invadens