Aphrosaurus is an extinct genus of plesiosaur from the Maastrichtian. The type species is Aphrosaurus furlongi (LACM 2748), named by Welles in 1943. The holotype specimen was discovered in the Moreno Formation in Fresno County, California in 1939 by rancher Frank C. Piava. A second specimen - LACM 2832 - was also found in the same formation and initially diagnosed as a juvenile of the same species, but has since been removed from the genus.

In 1943, Samuel Welles described Aphrosaurus furlongi along with several other plesiosaurs from the same fossil assemblage in Fresno County, California. Aphrosaurus was found below a different juvenile species, Morenosaurus stocki, within the Tierra Loma Member of the Panoche Hills. The Moreno Formation dates back to the early Maastrichtian, and is composed of depositional layers of turbidite, sandstone, and shale. It is part of the larger Chico Formation, which contacts the Panoche Formation and, during the Cretaceous, composed a sea shelf along the coast of California and the Pacific Ocean.

The diagnosis of Aphrosaurus as a unique species was initially determined by the presence of a deepened ventral notch on the centra of the cervical vertebrae, which was determined to be an autapomorphy of the species. LACM 2832, found in the same formation, was also initially determined to be Aphrosaurus with ontogenic features, but that classification was rejected in a reappraisal on the basis of systemic differences between the two specimens that likely could not be explained by ontogeny.

Welles took the name Aphrosaurus from the Greek for "sea foam" + "lizard", and furlongi in honor of University of California Berkeley field assistant and specimen preparator Eustace Furlong.

The axial skeleton of the holotype fossil is composed of 18 cervical, three pectoral, and 15 dorsal vertebrae, though Welles initially described "10 posterior cervical preceded by 11 indeterminate cervical and followed by 17 crushed dorsals." The diagnostic ventral groove is visible on the 14th to 6th prepectorals, and the prezygapophyses project anteriorly and meet. Only a few of the vertebrae retain the diapophyses, which slant ventrally in the more posterior half, though that may be an artifact of being crushed. Only three dorsal ribs are preserved, each different from each other, suggesting that they are each from a different section of the skeleton.

In addition to the axial skeleton, a good portion of the appendicular skeleton was also preserved. Both the pectoral and pelvic girdles are mostly complete, with a smooth clavicle-interclavicle complex lacking the keel and suture lines. The scapulae shape is indeterminate due to damage, but likely met in the midline. Two elements were initially labeled by Welles as the ilium, which was rectified when O'Gorman articulated both with the ischium and was able to classify one as a sacral rib instead.

Both fore and hind limbs were preserved, though both humeri and the left femur are very badly damaged. The left humerus has a large muscle scar on the ventral surface. The radius and ulna in both limbs are wider than long, a typical feature seen in plesiosaurus, and an epipodial foramen is present on the right forelimb despite Welles' initial analysis indicating none present. Welles noted that the right forelimb is severely distorted, and suggested that it was the result of a pathology sustained to the animal when it was alive. The hind limb description is based entirely on the right hind limb due to the damage to the left, and has no separation between the trochanter and capitulum. There is a raised muscle scar on the ventral surface.

No skull exists.