Ophelia borealis has a stout, segmented body that increases rapidly in width from the head to the 10th segment and grows to up to 2.3 cm in length. The body is divided into a head, thorax of 10 segments, abdomen of 20 segments and a pygidium. It is deep red in colour with an iridescent blue or lilac overtone. The head bears a pointed cone-shaped prostomium and a short, blunt proboscis. The head is differentiated from the thorax by 15 superficial rings. Segments are marked by 5-7 superficial rings, and a pair of parapodia on either side bearing cylindrical, slender chaetae that are longer dorsally. The abdomen bears slender chaetae-like gills and is distinguished by a deep ventral groove that runs from the 7th segment towards the pygidium and two lateral grooves that house vertical rows of small pores above the notopodia. Numerous papillae cover the surface of the pygidium.This species is similar to Ophelia limacina.