Kentrosaurus (/ˌkɛntroʊˈsɔːrəs/ KEN-troh-SOR-əs; lit. 'prickle lizard') is a genus of stegosaurid dinosaur from the Late Jurassic in Lindi Region of Tanzania. The type species is K. aethiopicus, named and described by German palaeontologist Edwin Hennig in 1915. Often thought to be a "primitive" member of the Stegosauria, several recent cladistic analyses find it as more derived than many other stegosaurs, and a close relative of Stegosaurus from the North American Morrison Formation within the Stegosauridae.

Fossils of K. aethiopicus have been found only in the Tendaguru Formation, dated to the late Kimmeridgian and early Tithonian ages, about 152 million years ago. Hundreds of bones were unearthed by German expeditions to German East Africa between 1909 and 1912. Although no complete skeletons are known, the remains provided a nearly complete picture of the build of the animal. In the Tendaguru Formation, it coexisted with a variety of dinosaurs such as the carnivorous theropods Elaphrosaurus and Veterupristisaurus, giant herbivorous sauropods Giraffatitan and Tornieria, and the dryosaurid Dysalotosaurus.

Kentrosaurus generally measured around 4–4.5 metres (13–15 ft) in length as an adult, and weighed about 700–1,600 kilograms (1,500–3,500 lb). It walked on all fours with straight hindlimbs. It had a small, elongated head with a beak used to bite off plant material that would be digested in a large gut. It had a, probably double, row of small plates running down its neck and back. These plates gradually merged into spikes on the hip and tail. The longest spikes were on the tail end and were used to actively defend the animal. There also was a long spike on each shoulder. The thigh bones come in two different types, suggesting that one sex was larger and more stout than the other.

The first fossils of Kentrosaurus were discovered by the German Tendaguru Expedition in 1909, recognised as belonging to a stegosaur by expedition leader Werner Janensch on 24 July 1910, and described by German palaeontologist Edwin Hennig in 1915. The name Kentrosaurus was coined by Hennig and comes from the Greek kentron/κέντρον, meaning "sharp point" or "prickle", and sauros/σαῦρος meaning "lizard", Hennig added the specific name aethiopicus to denote the provenance from Africa. Soon after its description, a controversy arose over the stegosaur's name, which is very similar to the ceratopsian Centrosaurus. Under the rules of biological nomenclature, forbidding homonymy, two animals may not be given the same name. Hennig renamed his stegosaur Kentrurosaurus, "pointed-tail lizard", in 1916, while Hungarian paleontologist Franz Nopcsa renamed the genus Doryphorosaurus, "lance-bearing lizard", the same year. If a renaming had been necessary, Hennig's would have had priority. However, because the spelling is different, both Doryphorosaurus and Kentrurosaurus are unneeded replacement names; Kentrosaurus remains the valid name for the genus with Kentrurosaurus and Doryphorosaurus being its junior objective synonyms.

Although no complete individuals were found, some material was discovered in association, including a nearly complete tail, hip, several dorsal vertebrae and some limb elements of one individual. These form the core of a mount in the Museum für Naturkunde by Janensch. The mount was dismantled during the museum renovation in 2006/2007, and re-mounted in an improved pose by Research Casting International. Some other material, including a braincase and spine, was thought to have been misplaced or destroyed during World War II. However, all the supposedly lost cranial material was later found in a drawer of a basement cupboard.

From 1909 onwards, Kentrosaurus remains were uncovered in four quarries in the Mittlere Saurierschichten (Middle Saurian Beds) and one quarry in the Obere Saurierschichten (Upper Saurian Beds). During four field seasons, the German Expedition found over 1200 bones of Kentrosaurus, belonging to about fifty individuals, many of which were destroyed during the Second World War. Today, almost all remaining material is housed in the Museum für Naturkunde Berlin (roughly 350 remaining specimens), while the museum of the Institute for Geosciences of the University of Tübingen houses a composite mount, roughly 50% of it being original bones.

In the original description, Hennig did not designate a holotype specimen. However, in a detailed monography on the osteology, systematic position and palaeobiology of Kentrosaurus in 1925, Hennig picked the most complete partial skeleton, today inventorised as MB.R.4800.1 through MB.R.4800.37, as a lectotype (see syntype). This material includes a nearly complete series of tail vertebrae, several vertebrae of the back, a sacrum with five sacral vertebrae and both ilia, both femora and an ulna, and is included in the mounted skeleton at the Museum für Naturkunde in Berlin, Germany. The type locality is Kindope, Tanzania, north of Tendaguru hill.

Unaware that Hennig had already defined a lectotype, Peter Galton selected two dorsal vertebrae, specimens MB.R.1930 and MB.R.1931, from the material figured in Hennig's 1915 description, as 'holotypes'. This definition of a holotype is not valid, because Hennig's selection has priority. In 2011, Heinrich Mallison clarified that all the material known to Hennig in 1915, i.e. all the bones discovered before 1912, when Hermann Heck concluded the last German excavations, are paralectotypes, and that MB.R.4800 is the correct lectotype.