Promachoteuthis sulcus is a species of promachoteuthid squid. It is distinguished from related taxa on the basis of several morphological features: nuchal fusion between the head and mantle, much larger size of arm suckers compared to club suckers, greater width of tentacle base than arm base, a recessed club base, and the presence of an aboral tentacle groove.
Promachoteuthis sulcus is known from a single specimen caught by the German research vessel R/V Walther Herwig in an open net off Tristan da Cunha, southern Atlantic Ocean (36°49′S 12°17′W / 36.817°S 12.283°W / -36.817; -12.283 (Promachoteuthis sulcus (only specimen caught))), at a depth of 1,750–2,000 m (5,740–6,560 ft).
