Acanthodoris pilosa, the hairy spiny doris,[3] is a species of sea slug, a dorid nudibranch, a shell-less marine gastropod mollusc in the family Onchidorididae.

At the head end are two rhinophores, much larger than the papillae, which house the sensory organs; these typically are bent towards the rear of the animal.

This sea slug has a uniform colour which may be anything from whitish to pale purplish-brown to darker brown or charcoal grey.

[7][8] It is common all round the British Isles in the shallow subtidal zone, at depths down to about 80 m (262 ft).

[5] Reports of this species from Japan[9] must be considered doubtful in the light of recent molecular phylogeny results which show that at least some of the animals from the North Pacific are Acanthodoris atrogriseata.