Dinocephalosaurus (meaning "terrible-headed reptile") is a genus of long necked, aquatic protorosaur that inhabited the Triassic seas of China. The genus contains the type and only known species, D. orientalis, which was named by Chun Li in 2003. Unlike other long-necked protorosaurs (which form a group known as the tanystropheids), Dinocephalosaurus convergently evolved a long neck not through elongation of individual neck vertebrae, but through the addition of neck vertebrae that each had a moderate length. As indicated by phylogenetic analyses, it belonged in a separate lineage that also included at least its closest relative Pectodens, which was named the Dinocephalosauridae in 2021. Like tanystropheids, however, Dinocephalosaurus probably used its long neck to hunt, utilizing the fang-like teeth of its jaws to ensnare prey; proposals that it employed suction feeding have not been universally accepted. It was probably a marine animal by necessity, as suggested by the poorly-ossified and paddle-like limbs which would have prevented it from going ashore.

Specimens belonging to the genus were first discovered in a locality near Xinmin in Guizhou, China in 2002. At the same locality, which dates to 244 million years ago, other marine reptiles such as Mixosaurus, Keichousaurus, and Wumengosaurus have also been found. While the type specimen consisted only of a skull and the very front of the neck, additional specimens soon revealed the complete form of the body. Further discoveries of Dinocephalosaurus specimens were made in Luoping, Yunnan, China, starting in 2008. At this locality, Dinocephalosaurus would have lived alongside Mixosaurus, Dianopachysaurus, and Sinosaurosphargis. One specimen discovered at the Luoping locality preserves an embryo within its abdomen, indicating that Dinocephalosaurus gave birth to live young like many other marine reptiles. Dinocephalosaurus is the only known member of the Archosauromorpha to give live birth, with the possible exception of the metriorhynchids, a group of marine crocodylomorphs.

The type specimen of Dinocephalosaurus was first discovered in 2002, during fieldwork conducted in Yangjuan Village, Xinmin District, Panxian County, Guizhou, China. It consists of a nearly-complete skull missing the left side of the jaw, as well as several associated cervical vertebrae. It was subsequently stored at the Institute of Vertebrate Paleontology and Paleoanthropology (IVPP) in Beijing, China under the collection number IVPP V13767, and a research paper describing the specimen was authored by IVPP paleontologist Chun Li and published by Acta Geologica Sinica in December 2003.

A second specimen discovered at the same locality represents a partially articulated skeleton that is only lacking the tail. Likewise stored at the IVPP, the specimen has the collection number IVPP V13898. The specimen was described in a brief correspondence authored by Li, Olivier Rieppel, and Michael LaBarbera that was published by Science in September 2004; a more detailed description was subsequently published by Rieppel, Li, Nicholas Fraser in a 2008 Journal of Vertebrate Paleontology paper.

The Panxian locality, from where these specimens originated, is part of Member II of the Anisian (Middle Triassic) Guanling Formation, which was initially thought to be 230 million years old, but has most recently been dated to 244 ± 1.3 million years old based on uranium–lead dating. Predominant deposits at this locality are composed of grey to dark grey marly limestone, as well as cherty limestone containing dolomite and bentonite beds. Dinocephalosaurus was specifically found in layer 90 of the Panxian deposits, a thin limestone layer which is traditionally assigned to the Upper Reptile Horizon (layers 87–90). Further below are the Middle Reptile (layers 81–85) and Lower Reptile (77–79) Horizons.

Subsequently, additional Dinocephalosaurus specimens were discovered from the slightly older Luoping locality, which has been dated to 245–244 million years old based on conodont biostratigraphy as well as preliminary radiometric dating. The specimens originate from bed 74 of the Luoping locality, in deposits located near the village of Dawazi, Luoping County, Yunnan, China. Bed 74 is part of a section composed of thin dark grey micritic limestone layers mixed with thicker layers of siliceous limestone and silty limestone, which extends downwards to bed 67.

One particular specimen is notable for containing an embryo in its abdominal region, of which cervical vertebrae, forelimbs, and several other elements are preserved. It was collected in 2008; by the time of its collection, weathering had already split the specimen into three blocks, with the gaps having been filled by modern soil. The specimen was then transferred to the Chengdu Center of the China Geological Survey, where it was prepared and stored under the collection number LPV 30280. Subsequently, a description of the specimen, authored by Jun Liu, Chris Organ, Michael Benton, Matthew Brandley, and Jonathan Aitchison, was published in February 2017 by Nature Communications.

At least one additional undescribed specimen of Dinocephalosaurus is known from an unspecified locality, having been catalogued as ZMNH M8752 in the Zhejiang Museum of Natural History. It was briefly mentioned in comparison to Fuyuansaurus by Fraser, Rieppel, and Li in 2013. Furthermore, another embryonic skeleton catalogued as IVPP V22788 was found at Luoping, and was considered by them to be closely related to Dinocephalosaurus on account of its large number of short neck vertebrae and its pillar-like limbs. However, they also noted some differences that were likely unrelated to growth, such as the presence of fewer (24 instead of 33) neck vertebrae and the presence of a scleral ring, which led them to assign it to not Dinocephalosaurus but a closely related animal.