Ohmdenosaurus ('Ohmden lizard') is a genus of sauropod dinosaur that lived during the Early Jurassic epoch in what is now Germany. The only specimen – a tibia (shinbone) and ankle – was discovered in rocks of the Posidonia Shale near the village of Ohmden. The specimen, which was originally identified as a plesiosaur, is exhibited in a local museum, the Urweltmuseum Hauff. In the 1970s, it caught the attention of German palaeontologist Rupert Wild, who recognised it as the remains of a sauropod. Wild named Ohmdenosaurus in a 1978 publication; the only known species is Ohmdenosaurus liasicus.

One of the earliest known sauropods, Ohmdenosaurus was quadrupedal (four-legged) and already had the columnar limbs typical for the group. It was small for a sauropod, with an estimated length of 3–4 m (10–13 ft). Its relationships to other sauropods remain uncertain due to the incompleteness of its remains, though one study concluded it was a eusauropod. The Posidonia Shale was deposited within a shallow inland sea and contains abundant and well-preserved fossils of marine reptiles, including ichthyosaurs and plesiosaurs. Ohmdenosaurus was a terrestrial animal, and the specimen must have been transported by predators or water currents at least 100 km (60 mi) from the shoreline to its site of burial. It is the only dinosaur fossil known from the shale.

The Posidonia Shale at Holzmaden in southwestern Germany is one of the world's major fossil Lagerstätten (fossil deposit of exceptional importance). Deposited within an inland sea, it contains abundant fossils of marine reptiles such as ichthyosaurs, plesiosaurs, and crocodyliforms, sometimes with soft tissue preservation. This organic-rich shale has been quarried for more than 400 years, first for roofing and paving, and later to also extract oil. Its exceptionally preserved fossils were made famous by Bernhard Hauff, who started to collect and prepare specimens found in a quarry owned by his father. In 1892, Hauff presented an ichthyosaur specimen that preserves the original body outline, revealing that ichthyosaurs possessed dorsal fins. Together with his son, he opened a local museum in Holzmaden in 1936/37, the Urweltmuseum Hauff, to display the finds.

In the 1970s, German palaeontologist Rupert Wild was visiting the Urweltmuseum Hauff when he noticed a fossil in a display labelled as the humerus (upper arm bone) of a plesiosaur. Wild recognised the specimen as a dinosaur fossil, borrowed the specimen for study and carried out further preparation. The fossil, which has no specimen number, consists of a right tibia (shinbone) together with the astragalus and calcaneus (the upper bones of the ankle). It had long been part of the museum's collection, having been collected from one of the early quarries near the village of Ohmden that were later refilled; the exact discovery site is unknown. In a 1978 publication, Wild determined that the dinosaur fossil belonged to a new genus and species, which he named Ohmdenosaurus liasicus. The generic name Ohmdenosaurus is derived from the village of Ohmden and from the Ancient Greek σαῦρος (sauros), meaning lizard or reptile. The specific name liasicus refers to the Lias, an old name for the Lower Jurassic of Europe.

A chunk of rock – a gray-black, finely laminated slate containing small fragments of fish fossils – is still attached to the lower end of the fossil. This rock indicates that the fossil was found in the Unterer Schiefer ('lower slate'), the oldest part of the Posidonia Shale. It is therefore early Toarcian in age (ca. 182 million years ago). When Ohmdenosaurus was described in 1978, it was one of the earliest sauropods known at the time and only the second fossil of a terrestrial saurian to be discovered from the Toarcian. Between the tibia and astragalus, the specimen also preserves a limestone geode that is rich in the mineral pyrite and contains fossils of the snail Coelodiscus.

Like all sauropods, Ohmdenosaurus was a quadrupedal (four-legged) herbivore with a long neck and tail. Wild estimated the total body length at 3–4 m (10–13 ft), which is relatively small for a sauropod.

The tibia is 405 mm (15.9 in) long. The bone preserves projections that served as attachment sites for muscles, including the cnemial crest, which projects by about 4 cm (1.6 in) from the upper front of the bone, and the crista lateralis, which runs for about 13 cm (5.1 in) down the upper half of the shaft but is mostly broken off. The upper end of the tibia is oval in shape when viewed from above, with a width-to-length ratio of 1.4. The lower end of the tibia is rotated by 90° relative to the upper end. The lower end of the tibia is formed by two rounded, well-separated prominences, the medial (inner) and lateral (outer) condyles. The medial condyle is much larger than the (broken) lateral condyle and located c. 2–3 cm (0.8–1.2 in) lower than the latter.

The astragalus is 140 mm (5.5 in) in diameter, sandal-shaped, and rotated by 90° out of its original position, exposing its upper surface. This surface has two concavities, a larger medial and a smaller lateral one; the latter was located about 2 cm (0.8 in) higher than the former. These concavities received the medial and lateral condyles of the tibia, respectively. A furrow between these concavities is thought to have been an attachment site for ligaments of the ankle. The much smaller calcaneus is 43 mm (1.7 in) in diameter and 15 mm (0.6 in) in maximal height, and circular in shape. Its probable lower surface is convex, and its probable upper surface is roughly textured, indicating the presence of a cartilaginous covering. Below the lateral condyle of the tibia, Wild noted several other small elements 5–30 mm (0.2–1.2 in) in size, which he identified as cartilage given their grainy and irregular surfaces. Unlike bone, cartilage is rarely preserved in fossils, and in this case might have been preserved thanks to the absorption of calcium salts. As these elements are located close to the attachment site of the Achilles tendon, the area of the ankle that experienced the highest stresses in life, Wild argued that they may represent sesamoids (small structures embedded within tendons). Alternatively, they could be calcified pieces of the cartilage of the astragalus.