Animal up to 9 cm long and 1-5 mm broad, with a rather stout, somewhat dorsoventrally compressed, body which gradually tapers posteriorly to the bluntly pointed tail. The head is distinct and shaped like a rounded triangle or shield. It bears two pairs of cephalic furrows. The anterior pair, dorsally incomplete, possesses short forwardly directed parallel ridges running perpendicular to the transverse furrows. A median longitudinal ridge or swelling runs from the tip of the head to the most anterior part of the body, where the posterior cephalic furrows meet to form a wide V-shape with the apex pointing caudally. The posterior furrows do not reach the mid-line on the ventral surface and are not as distinct as the anterior pair, especially in pale coloured worms (N. pulcher-detail).
The eyes are distributed irregularly near the lateral cephalic margins, mostly in front of the anterior cephalic furrows. Their number increases with age; in young individuals of less than 10 mm length, 10-20 large eyes were found, in contrast to older specimens of 40-50 mm long, which had 70-80 smaller eyes. Irrespective of eye number, their arrangement is remarkably constant.
Dorsally Nipponnemertes pulcher is coloured brown, red or pink, but the lateral and ventral surfaces are always much lighter and may even appear completely unpigmented. The dorsal longitudinal ridge on the head is sometimes darker than the rest of the body, and the ridge is emphasized through the absence of pigmentation from its lateral margins. The body colouration is affected by age, degree of sexual maturation and habitat. Juveniles and mature males are lighter in hue than the dark red colour of the ovaries in gravid females, and individuals maintained in an aquarium for some time tend to develop a deeper colouration. Ova develop at the beginning of May and are ripe towards the end of June.
The accessory stylets are often liberated prematurely from the proboscis in this species, with the result that additional stylets are quite often found embedded in the stylet basis.
In the North Sea Nipponnemertes pulcher has been found sublittorally among corallines or on coarser sediments such as sand, gravel or shelly debris at depths down to 240 m. In other areas it is also commonly found on muddy or stony bottoms and as deep as 569 m. Very occasionally it may occur beneath stones at the extreme lower levels of the shore.
It has a wide geographic range. In the northern hemisphere it is reported from the east coast of North America, Greenland, the Faeroe Islands, the White Sea and in northern Europe from the Atlantic coast of France to Scandinavia. In the southern hemisphere it is known from Chile and many parts of the Antarctic and sub-Antarctic areas.