Cetiosauriscus (/ˌsiːtioʊˈsɔːrɪskəs/ SEE-tee-oh-SOR-iss-kəs) is a genus of sauropod dinosaur that lived between 166 and 164 million years ago during the Callovian (Middle Jurassic Period) in what is now England. A herbivore, Cetiosauriscus had – by sauropod standards – a moderately long tail, and longer forelimbs, making them as long as its hindlimbs. It has been estimated as about 15 m (49 ft) long and between 4 and 10 t (3.9 and 9.8 long tons; 4.4 and 11.0 short tons) in weight.

The only known fossil includes most of the rear half of a skeleton as well as a forelimb (NHMUK PV R3078). Found in Cambridgeshire in the 1890s, it was described by Arthur Smith Woodward in 1905 as a new specimen of the species Cetiosaurus leedsi. This was changed in 1927, when Friedrich von Huene found NHMUK PV R3078 and the C. leedsi type specimen to be too different from Cetiosaurus, warranting its own genus, which he named Cetiosauriscus, meaning "Cetiosaurus-like". Cetiosauriscus leedsi was referred to the sauropod family Diplodocidae because of similarities in the tail and foot, and had the dubious or indeterminate species "Cetiosauriscus" greppini, "C." longus, and "C." glymptonensis assigned to it. In 1980, Alan Charig named a new species of Cetiosauriscus for NHMUK PV R3078 because of the lack of comparable material to the type of C. leedsi; this species was named Cetiosauriscus stewarti. Because of the poor state of preservation of the Cetiosauriscus leedsi fossil, Charig sent a petition to the International Commission on Zoological Nomenclature to instead make C. stewarti the type species. Cetiosauriscus stewarti became the oldest confirmed diplodocid until a phylogenetic analysis published in 2003 instead found the species to belong to Mamenchisauridae, and followed by studies in 2005 and 2015 that found it outside Neosauropoda, while not a mamenchisaurid proper.

Cetiosauriscus was found in the marine deposits of the Oxford Clay Formation alongside many different invertebrate groups, marine ichthyosaurs, plesiosaurs and crocodylians, a single pterosaur, and various dinosaurs: the ankylosaur Sarcolestes, the stegosaurs Lexovisaurus and Loricatosaurus, the ornithopod Callovosaurus, as well as some unnamed taxa. The theropods Eustreptospondylus and Metriacanthosaurus are known from the formation, although probably not from the same level as Cetiosauriscus.

The fossil later known as Cetiosauriscus was originally ascribed to the genus Cetiosaurus—one of the first sauropods to be named, in 1842 by palaeontologist Richard Owen, and one with a complicated history due to many unfounded referrals of species and specimens, involving almost all English sauropod specimens. The type species of Cetiosaurus has changed throughout history because of incomplete remains and the taxon's significance, and many aspects of its anatomy and relationships are still uncertain. Cetiosaurus was originally named to include C. medius, C. brevis, C. brachyurus and C. longus, which span from the Middle Jurassic to the Early Cretaceous of various localities across England. As none of these species are truly diagnostic, and Cetiosaurus is a historically and taxonomically important taxon, the more complete Middle Jurassic species C. oxoniensis named by geologist John Phillips in 1871 became the type species. C. glymptonensis was also named in the same publication by Phillips, but is less complete and of questionable validity.

Another English taxon, Ornithopsis hulkei, was named in 1870 by palaeontologist Harry Govier Seeley for vertebrae from the Early Cretaceous Wessex Formation, younger than the existing species of Cetiosaurus. Seeley considered Ornithopsis to be closely related to Cetiosaurus, but different due to the internal bone structure. An additional species, Ornithopsis leedsii was named in 1887 by John Hulke for a pelvis, vertebrae and ribs collected by Alfred Nicholson Leeds, an English farmer and amateur fossil collector who throughout his life compiled numerous collections of fossils from the Oxford Clay. O. leedsii, from the Late Jurassic, showed similarities to older Cetiosaurus oxoniensis as well as younger O. hulkei. It was described in more detail by Seeley in 1889, where he considered O. hulkei, C. oxoniensis and O. leedsii to all be in the same genus, bearing the name Cetiosaurus. But naturalist Richard Lydekker discussed with Seeley, before the publication of Seeley's 1889 paper, that Cetiosaurus and Ornithopsis were not the same taxon. Lydekker suggested that Wealden fossils (including O. hulkei) belonged to Ornithopsis and the Jurassic remains (including O. leedsii and C. oxoniensis) to Cetiosaurus. Lydekker in 1895 changed his mind and referred the species O. leedsii to Pelorosaurus (known already from the species P. brevis, once named Cetiosaurus brevis)—as P. leedsi—and referred the genus to Atlantosauridae. Lydekker's classification of the species was not supported by later authors like palaeontologist Arthur Smith Woodward in 1905, who followed Seeley's classification scheme.

The sauropod fossil today known as Cetiosauriscus stewarti was discovered in May 1898 by clay workers in the area around Fletton to the south of Peterborough and east of the Great Northern Railway line. Pits in this region expose the fossil-rich sedimentary rocks of the marine Oxford Clay, which is of middle Callovian age and today regarded as one of the classic geological formations of British palaeontology. The sauropod fossil possibly stems from NPBCL pit No. 1, which was the northernmost pit operated by the New Peterborough Brick Company Limited, and which produced the most vertebrate fossils. The discovery was brought to the attention of Leeds, who, after excavation, took the sauropod specimen to Eyebury, the Leeds' family home. In mid-August, after some cleaning and repairing of the specimen, geologist Henry Woodward visited Eyebury and produced a life-sized drawing of the remains for presentation at the British Association for the Advancement of Science Meeting. Following this presentation, on 17 August 1898, Henry Woodward returned with American palaeontologist Othniel Charles Marsh, who considered the sauropod to be closely related to the North American taxon Diplodocus. Alfred Leeds offered the sauropod to the British Museum of Natural History (BMNH, now abbreviated as NHMUK) for £250, which would equate to about £30,529 in 2017. The NHMUK had earlier in 1890 and 1892 bought the First and Second Collections of Alfred Leeds, respectively. Woodward, Keeper of Geology at the NHMUK, had "great pleasure" to recommend to the Trustees of the NHMUK the fossil be purchased. The purchase was sanctioned on 25 February 1899, along with the purchase of assorted other remains for just over £357 (~£43,596 in 2018), where the Leeds sauropod gained the accession number BMNH R3078 (now NHMUK PV R3078).

The amount of material made NHMUK PV R3078 the most complete sauropod specimen from the United Kingdom, comparable only later to the "Rutland Dinosaur" (referred to Cetiosaurus) discovered in 1967. Known regions of the specimen include the forelimb, hindlimb and vertebral column. The forelimb lacks the manus (hand) and part of the radius and ulna, although the hindlimb lacks only a few bones in the pes (foot) and fragments of the tibia, fibula and ilium. The vertebrae known are four parts of dorsal vertebrae, the neural spines of the sacrum, multiple anterior caudal vertebrae (tail bones), and a series of 27 nearly complete vertebrae from the middle of the tail with associated or articulated chevrons (ribs along the underside of the tail), although the vertebral series is not continuous. A tail tip (NHMUK PV R1967) from the same locality, but a different individual was thought by palaeontologist Alan Charig in 1980 to belong to Cetiosauriscus. The assignment of NHMUK PV R1967 to Cetiosauriscus was considered unlikely in alternate studies by palaeontologists Friedrich von Huene, Paul Upchurch and Darren Naish because of the lack of overlap and uncertain phylogenetic positions. In 1903, the skeleton was mounted as preserved in the British Museum, so it could be more easily compared with other mounted sauropods from North America. The mount of Cetiosauriscus was put on display just prior to the cast skeleton of Diplodocus, and was displayed with the dorsal vertebrae NHMUK PV R1984 and some isolated teeth from a camarasaurid (possibly referable to Cetiosauriscus), making it the first sauropod skeleton mounted in the United Kingdom.

NHMUK PV R3078 was referred in 1905 by Arthur Woodward to the species Cetiosaurus leedsii, as it was from the same geologic formation as other specimens that were assigned to C. leedsii. Woodward also referred the dorsal vertebrae NHMUK PV R1984 and the tail tip NHMUK PV R1967 to the species. In 1927, Huene briefly described the anatomy of the species C. leedsii, where he noted that it shared many similarities with Haplocanthosaurus and was most likely between Cetiosaurus proper and the former genus. For this reason, Huene proposed the new genus name Cetiosauriscus for the species. To the genus he referred the specimens NHMUK PV R1984–R1988 and NHMUK PV R3078.