Ichthyosauria is an order of large extinct marine reptiles sometimes referred to as "ichthyosaurs", although the term is also used for wider clades in which the order resides.

Ichthyosaurians thrived during much of the Mesozoic era; based on fossil evidence, they first appeared around 250 million years ago (Ma) and at least one species survived until about 90 million years ago, into the Late Cretaceous. During the Early Triassic epoch, ichthyosaurs and other ichthyosauromorphs evolved from a group of unidentified land reptiles that returned to the sea, in a development similar to how the mammalian land-dwelling ancestors of modern-day dolphins and whales returned to the sea millions of years later, which they gradually came to resemble in a case of convergent evolution. Ichthyosaurians were particularly abundant in the Late Triassic and Early Jurassic periods, until they were replaced as the top aquatic predators by another marine reptilian group, the Plesiosauria, in the later Jurassic and Early Cretaceous, though previous views of ichthyosaur decline during this period are probably overstated. Ichthyosaurians diversity declined due to environmental volatility caused by climatic upheavals in the early Late Cretaceous, becoming extinct around the Cenomanian-Turonian boundary approximately 90 million years ago.

Scientists became aware of the existence of ichthyosaurians during the early 19th century, when the first complete skeletons were found in England. In 1834, the order Ichthyosauria was named. Later that century, many finely preserved ichthyosaurian fossils were discovered in Germany, including soft-tissue remains. Since the late 20th century, there has been a revived interest in the group, leading to an increased number of named ichthyosaurs from all continents, with over fifty genera known.

Ichthyosaurian species varied from 1 to 26 metres (3 to 85 ft) in length. Ichthyosaurians resembled both modern fish and dolphins. Their limbs had been fully transformed into flippers, which sometimes contained a very large number of digits and phalanges. At least some species possessed a dorsal fin. Their heads were pointed, and the jaws often were equipped with conical teeth to catch smaller prey. Some species had larger, bladed teeth to attack large animals. The eyes were very large, for deep diving. The neck was short, and later species had a rather stiff trunk. These also had a more vertical tail fin, used for a powerful propulsive stroke. The vertebral column, made of simplified disc-like vertebrae, continued into the lower lobe of the tail fin. Ichthyosaurians were air-breathing, warm-blooded, and bore live young. Many, if not all, species had a layer of blubber for insulation.

Like other ancient marine reptiles, such as those in the clades Mosasauria and Plesiosauria, the genera in Ichthyosauria are not part of the clade Dinosauria.

The first known illustrations of ichthyosaur bones, vertebrae, and limb elements were published by Edward Lhuyd in his Lithophylacii Brittannici Ichnographia of 1699. Lhuyd thought that they represented fish remains. In 1708, the Swiss naturalist Johann Jakob Scheuchzer described two ichthyosaur vertebrae assuming they belonged to a man drowned in the Universal Deluge. In 1766, an ichthyosaur jaw with teeth was found at Weston near Bath. In 1783, this piece was exhibited by the Society for Promoting Natural History as those of a crocodilian. In 1779, ichthyosaur bones were illustrated in John Walcott's Descriptions and Figures of Petrifications. Towards the end of the eighteenth century, British fossil collections quickly increased in size. Those of the naturalists Ashton Lever and John Hunter were acquired in their totality by museums; later, it was established that they contained dozens of ichthyosaur bones and teeth. The bones had typically been labelled as belonging to fish, dolphins, or crocodiles; the teeth had been seen as those of sea lions.

The demand by collectors led to more intense commercial digging activities. In the early nineteenth century, this resulted in the discovery of more complete skeletons. In 1804, Edward Donovan at St Donats uncovered a four-metre-long (13 ft) ichthyosaur specimen containing a jaw, vertebrae, ribs, and a shoulder girdle. It was considered to be a giant lizard. In October 1805, a newspaper article reported the find of two additional skeletons, one discovered at Weston by Jacob Wilkinson, the other, at the same village, by Reverend Peter Hawker. In 1807, the last specimen was described by the latter's cousin, Joseph Hawker. This specimen thus gained some fame among geologists as 'Hawker's Crocodile'. In 1810, near Stratford-upon-Avon, an ichthyosaur jaw was found that was combined with plesiosaur bones to obtain a more complete specimen, indicating that the distinctive nature of ichthyosaurs was not yet understood, awaiting the discovery of far better fossils.

In 1811, in Lyme Regis, along the Jurassic Coast of Dorset, the first complete ichthyosaur skull was found by Joseph Anning, the brother of Mary Anning (who in 1812 while still a young girl,) secured the torso of the same specimen. Their mother, Molly Anning, sold the combined piece to squire Henry Henley for £23. Henley lent the fossil to the London Museum of Natural History of William Bullock. When this museum was closed, the British Museum bought the fossil for a price of £47/5s;^{[verification needed]} it still belongs to the collection of the independent Natural History Museum and has the inventory number NHMUK PV R1158 (formerly BMNH R.1158). It has been identified as a specimen of Temnodontosaurus platyodon.