Nectocaris is a genus of squid-like animal of controversial affinities known from the Cambrian period. The initial fossils were described from the Burgess Shale of Canada. Other similar remains possibly referrable to the genus are known from the Emu Bay Shale of Australia and Chengjiang Biota of China.

Nectocaris was a free-swimming, predatory or scavenging organism. This lifestyle is reflected in its binomial name: Nectocaris means "swimming shrimp" (from the Ancient Greek νηκτόν, nekton, meaning "swimmer" and καρίς, karis, "shrimp"). Two morphs are known: a small morph, about an inch long, and a large morph, anatomically identical but around four times longer.

Nectocaridids had controversial affinities before 2025. Some authors have suggested that they represent the earliest known cephalopods. However, their morphology is strongly dissimilar to confirmed early cephalopods, and thus their affinities to cephalopods and even to molluscs more broadly are rejected by many authors. Their affinities to any animal group beyond Bilateria were long considered uncertain, until a 2025 study, which interpreted them as relatives of modern chaetognaths (arrow worms).

Nectocaris had a flattened, kite-shaped body with a fleshy fin running along the length of each side. The small head had two stalked eyes, a single pair of tentacles, and a flexible funnel-shaped structure opening out to the underside of the body. The funnel often gets wider away from the head. The funnel has been suggested to represent an eversible (able to be turned inside out) pharynx. Internally, a long cavity runs along the body axis, which is suggested to represent the digestive tract. The body contains a pair of gills; the gills comprise blades emerging from a zig-zag axis. Muscle blocks surrounded the axial cavity, and are now preserved as dark blocks in the lateral body. The fins also show dark blocks, with fine striations superimposed over them. These striations often stand in high relief above the rock surface itself.

Although Nectocaris is known from Canada, China and Australia, in rocks spanning some 20 million years, there does not seem to be much diversity; size excepted, all specimens are anatomically very similar. Historically, three genera have been erected for nectocaridid taxa from different localities, but these 'species' – Petalilium latus and Vetustovermis planus – likely belong to the same genus or even the same species as N. pteryx. Within N. pteryx, there seem to be two discrete morphs, one large (~10 cm in length), one small (~3 cm long). These perhaps represent separate male and female forms.

The unusual shape of the nectocaridid funnel has led to its interpretation as an eversible proboscis. Martin R. Smith and Jean-Bernard Caron have suggested that it was used for jet propulsion, though this has been questioned by other authors. The eyes of Nectocaris would have had a similar visual acuity to modern Nautilus (if they lacked a lens) or squid (if they did not). They are thought to have been freely-swimming nektonic organisms, that were either scavengers or predators on soft-bodied animals, using their tentacles to manipulate food items.

The affinity of Nectocaris is controversial. Martin R. Smith and Jean-Bernard Caron have suggested that nectocaridids represent early cephalopods. In a 2010 publication in Nature, they suggested that the ancestor of modern cephalopods and nectocaridids probably lacked a mineralised shell, while Smith in a later 2013 publication suggested that it may be more plausible that nectocaridids had instead lost a mineralised shell and developed a morphology convergent on modern coleoids, and in 2020 Smith described resembling taxon Nectocotis from Ordovician, which is suggested to have had an internal skeletal element. However, other authors contend that the morphology of nectocaridids is contrary to what is known about cephalopod and mollusc evolution, and they cannot be accommodated within these groups, and can only be confidently placed as members of Bilateria. A 2025 study based on specimens of the related Nektognathus from Sirius Passet of Greenland which preserved neural tissue showed the presence of a ventral ganglion (part of the nervous system within the body cavity), a distinctive charateristic found only among arrow worms (chaetognaths) as well as other extinct Cambrian relatives like the large Amiskwia also known from the Burgess Shale, firmly placing nectocaridids in the chaetognath stem-group. Due to lack of diagnostic features, authors have not included Nectocotis to nectocaridid.

Nectocaris has a long and convoluted history of study. Charles Doolittle Walcott, the discoverer of the Burgess Shale, had photographed the one specimen he had collected in the 1910s, but never had time to investigate it further. As such, it was not until 1976 that Nectocaris was formally described, by Simon Conway Morris.