Arion lusitanicus, also known by its common name Portuguese slug, is a species of air-breathing land slug, a terrestrial pulmonate gastropod mollusk in the family Arionidae.

It is a rather large (up to 80 mm (3.1 in) long) slug of reddish-brown colour.

The margin of the foot is reddish or yellowish with dark transverse lines reaching into the lateral parts of the sole.

The sole is olive greyish with darker marginal zones.

[2] The more well known Spanish slug Arion vulgaris (Moquin-Tandon, 1855) was for a time misidentified as Arion lusitanicus, but the two slugs are not very closely related, differing in internal anatomy, shape of spermatophore and number of chromosomes.