A new nematode species, Theristus polychaetophilus, is described as an external parasite of the spionid polychaete Scolelepis (Scolelepis) squamata. Morphological characteristics of the nematode, its attraction to the polychaete, and its ability to alter polychaete tissue are regarded as evidence of parasitism. The nematode is a member of the "flevensis" group of Theristus species, and is distinguished by the pronounced "pharyngeal" musculature, low labial papillae, sclerotized buccal cavity ring, and vulval flap.