Chacaicosaurus is a genus of neoichthyosaurian ichthyosaur known from the Middle Jurassic of Argentina. The single known specimen of this genus was excavated from the Los Molles Formation in Neuquén Province, and is housed at the Museo Olsacher under the specimen number MOZ 5803. This specimen consists of a skull, forelimb, some vertebrae, and some additional postcranial elements. The genus was named by Marta Fernández in 1994, and contains a single species, Chacaicosaurus cayi, making it the first named distinctive ichthyosaur from the Bajocian stage. It is a medium-sized ichthyosaur with a very long snout, which bears a ridge running along each side. The forelimbs of Chacaicosaurus are small and contain four main digits.

Different authors have classified Chacaicosaurus in different ways; some consider it a thunnosaur closely related to the ophthalmosaurids, others instead place it outside of Thunnosauria, often near Hauffiopteryx. However, as it is very similar to Stenopterygius, some researchers instead classify it within that genus as S. cayi, a placement originally suggested by Fernández in 2007. The only known specimen of Chacaicosaurus appears to be an adult based on the shape of its limb bones. Chacaicosaurus inhabited open marine waters which it shared with the ophthalmosaurid ichthyosaur Mollesaurus as well as a plesiosaur, a thalattosuchian, and various invertebrates.

In 1990, Zulma Gasparini led an excavation team from the Museo Olsacher and the Museo de La Plata that operated in the Neuquén Basin of Argentina in Patagonia. They recovered a partial skeleton of an ichthyosaur from the Chacaico Sur locality of the Los Molles Formation, in Zapala Department, Neuquén Province. This specimen, given the number MOZ (standing for Museo Olsacher) 5803, includes a complete but damaged skull, six vertebrae, one bone from the shoulder girdle, the upper end of a humerus (upper arm bone), an articulated forelimb, the upper end of a femur (thighbone), and some additional phalanges (digit bones).

Marta S. Fernández named the new genus and species Chacaicosaurus cayi in 1994 to contain MOZ 5803, which serves as its holotype and is the only known specimen of the genus. The name of the genus is derived from the name of the Chacaico Sur locality and sauros, Greek for "lizard", while that of the species refers to the Mapuche sea god Cay. Very few fossils of ichthyosaurs from the Aalenian to Bathonian stages of the Middle Jurassic are known, compared to the glut of Early Jurassic ichthyosaur material that has been discovered. C. cayi was the first valid ichthyosaur from this time interval to be named, with all previously named species based on remains to poor to be distinctive. Further ichthyosaurs of this time have since been found, including Mollesaurus periallus from the same time and place as Chacaicosaurus and the Aalenian German Stenopterygius aaleniensis.

Chacaicosaurus is a medium-sized ichthyosaur based on the size of its skull; it appears to be significantly larger than Stenopterygius. The long-snouted skulls of ichthyosaurs housed enlarged eyes. The limbs of ichthyosaurs were modified into rigid flippers, and the bodies of Jurassic ichthyosaurs were streamlined and spindle-shaped, with very short necks. A downward bend in the tail supported a crescentic tail fin, and a dorsal fin constructed of soft tissue was present on the back.

The skull of the holotype measures 98 centimeters (3 ft 3 in) long, while its mandible (lower jaw bones) is 99 centimeters (3 ft 3 in) long. The skull bears a narrow snout which, characteristically, is heavily elongated, making up 80% of the skull's length, gradually sloping to a point at its front. Uniquely among ichthyosaurs, the snout bears elongate, rounded ridges that run longitudinally along the premaxillae and nasals, with one ridge on each side. The cranial proportions of Chacaicosaurus are similar to those of other long-snouted ichthyosaurs, such as Eurhinosaurus, though unlike that genus Chacaicosaurus does not have an overbite. Unusually, adult Chacaicosaurus appear to have either had very reduced teeth or been toothless.

The enlarged external nares (openings for the nostrils) of Chacaicosaurus each measure about 10 centimeters (3.9 in) long, and are not positioned especially close to the orbits (eye openings). The orbits of Chacaicosaurus are very poorly preserved, so their size is unclear. In the description of the genus, Fernández estimated the scleral ring (the ring of bony plates supporting the eyeball) to have had a diameter of around 13 centimeters (5.1 in). She considered this to also be a reasonable rough approximation of the width of the orbit, thus concluding that Chacaicosaurus had especially small orbits. While Ryosuke Motani also stated that the orbits were small in 1999, Michael Maisch and Andreas Matzke in 2000 instead considered the orbits to be especially large.

The wide basioccipital (rear lower braincase bone) of Chacaicosaurus lacks a peg-like projection on its front end. The surfaces on the basioccipital for articulation with two pairs of other braincase bones, the opisthotics and stapedes, are both angled towards the top of the skull, with the latter pair reclined. The occipital condyle, the knob on the back of the skull for the articulation of the spinal column, is clearly demarcated from the rest of the basioccipital. The occipital condyle is not especially large, taking up relatively little of the rear face of the basioccipital, with additional bone surface extending outwards and beneath it.