The type and only known species, Nectosaurus halius, was found in the Hosselkus Limestone and described by John C. Merriam in 1905, making it one of the first thalattosaurians known (along with Thalattosaurus).

The holotype, UCMP 9124, is an incomplete skeleton including vertebrae, a humerus, coracoid, ulna, radius, and partial skull and mandibles.

[2] Based on the position of the vomers, the general shape of the premaxillae was inferred to be that of a dramatically downward hooking rostrum, descending at a vertical angle.

[2] Nectosaurus was a member of a group of marine reptiles known as thalattosaurs, characterized by their long, paddle-like tails and short legs with independently movable digits.

[3] Although a 2001 analysis considered it a close relative of Xinpusaurus and Paralonectes because it was interpreted as having an upward-curving maxilla, further inquiry has shown that this was mistaken.