Brancasaurus (meaning "Branca's lizard") is a genus of plesiosaur which lived in a freshwater lake in the Early Cretaceous of what is now North Rhine-Westphalia, Germany.

With a long neck possessing vertebrae bearing distinctively-shaped "shark fin"-shaped neural spines, and a relatively small and pointed head, Brancasaurus is superficially similar to Elasmosaurus, albeit smaller in size at 3.26 metres (10.7 ft) in length as a subadult.

The type species of this genus is Brancasaurus brancai, first named by Theodor Wegner in 1914 in honor of German paleontologist Wilhelm von Branca.

Over time, a number of parts have been lost, including several pieces of the skull, teeth, gastralia and caudal vertebrae, a second femur, and a radius, tibia, and fibula.

[2] The clay pit from which the type specimen originates is part of the Isterberg Formation in the Bückeberg Group,[3] also known in the past as the "German Wealden facies".

[2] E. Koken named Plesiosaurus limnophilus in 1887 based on isolated cervical vertebrae from outcrops of the Bückeberg Group in Lower Saxony.

A narrow, rounded ridge along the middle of the top surface of the skull extends from near the front of the premaxilla to the back of the eye sockets.

The squamosal bones arch around to form the curved back of the skull, and bear a ridge on top for attachment of neck muscles.

On the imperfectly-preserved lower jaw, the coronoid eminence seems to be relatively low, judging by the narrow and slightly curved top edge of the surangular bone.

There are three pectoral vertebrae at the neck-body transition, which are weakly concave, taller than they are long, and have rectangular-shaped neural spines that are directed slightly backwards.

The scapulae have prominent shelves on each side (diagnostic of leptocleidids and polycotylids, but not strongly differentiated in elasmosaurids), and their glenoids are clearly concave, with roughened attachments for cartilage.

The regions where the coracoids contact is vaulted and thickened to form a weak, ridge-like projection, comparable to but probably convergently acquired from elasmosaurids.

[9] The ilia are rod-shaped and bent, with blunt projections halfway along their outer rims; at the top end, they are flattened into a fan-like shape.

Covering the limbs and the rest of the body was a layer of smooth, multilayered calcite, which was originally interpreted as preservation of decaying skin.

[12] Nevertheless, a number of contrary taxonomic opinions have been expressed; in particular, Theodore E. White created a new family, Brancasauridae, to contain Brancasaurus, Seeleyosaurus, and "Thaumatosaurus", a defunct genus with species now belonging to Rhomaleosaurus and Meyerasaurus.

[15][2] An alternative phylogenetic hypothesis that has gained substantial traction places Brancasaurus in the clade Leptocleididae,[16][8][17] along with other leptocleidids including Leptocleidus itself, Vectocleidus, Umoonasaurus, Nichollssaura, and also possibly Hastanectes.

In addition to the fact that the type specimen is a subadult, this inconsistency in results can be attributed to the mix of leptocleidid, polycotylid, and elasmosaurid characteristics that is seen in Brancasaurus.

The Bückeberg Group, from which Brancasaurus originates, likely represented a large, continental freshwater lake that the surrounding uplands drained into.