The Celtic sea slug has a solid, oval and fleshy body up to 13 mm in length and 6 mm in width. It is a dark green-black inconspicuous slug with a mantle that is covered in large, coarse, evenly spaced tubercles, to which sand grains often adhere. When Onchidella celtica is motionless, the mantle completely obscures the head and pale grey foot. The head bears two short, thick, cylindrical tentacles with eyes at the tips. The head is only visible when the Celtic sea slug is mobile.