Protoichthyosaurus is a genus of ichthyosaur from the early Jurassic of southern England and possibly Switzerland. Two species are known, P. prostaxalis—the type species, named by Appleby in 1979—and P. applebyi. A third species, P. prosostealis, was named by Appleby, but it was removed from the genus in 2017 due to its similarity to Ichthyosaurus. The genus Protoichthyosaurus was synonymized with Ichthyosaurus by Maisch and Hungerbuhler in 1997, and again by Maisch and Matzke in 2000. However, it was found to be distinct in 2017 by Dean Lomax and colleagues, who separated it from Ichthyosaurus on account of differences in the arrangement and shape of the carpal ossifications, as well as the absence of the fifth digit. The species most likely lived during the Hettangian stage, but may have lived as early as the Rhaetian and as late as the Sinemurian.

Species belonging to the genus were medium-sized, with P. prostaxalis measuring no more than 2.5 metres (8 ft 2 in) in length and P. applebyi reaching 2 metres (6 ft 7 in) at most. P. prostaxalis can be distinguished from P. applebyi and from other ichthyosaurs by the large, tall, and triangular maxilla that extends beyond the nasal bones at its front end; a vertically short but thick postorbital bone; and the lacrimal bone having an upward projection longer than its forward projection. Meanwhile, P. applebyi can be distinguished by the narrow, crescent-shaped postorbital; the low maxilla; the nasal reaching to the front of the maxilla; the lacrimal having a forward projection the same length as or longer than the upward projection; and the presence of a plate-like upward projection on the humerus.

In 1979, Robert M. Appleby noted that some ichthyosaur foreflippers showed unique configurations of bones, and named a new genus, Protoichthyosaurus, for these specimens. He considered the known remains to represent two species; the type species P. prostaxalis and the additional species P. prosostealis. Appleby chose BRLSI M3553 (at the time under the specimen number B. 1963'5/OS), the front part of a skeleton from Street, Somerset, to be the holotype of P. prostaxalis. Appleby assigned an additional four specimens to this species: the partial skeletons BRLSI M3555 and BRLSI M3563, the nearly complete skeleton OUMNH J.13799, and the isolated flipper LEICT G454.1951/164. Appleby included only one specimen in P. prosostealis, the well-preserved skeleton BRLSI M3572 (formerly B. 1963'24/OS).

However, in 1997, Michael W. Maisch and Axel Hungerbühler synonymized Protoichthyosaurus with Ichthyosaurus, a position reiterated by Maisch later that year and supported by Christopher McGowan and Ryosuke Motani in 2003. In 2000, Maisch and Andreas Matzke elaborated on this synonymy, noting that the foreflippers of Ichthyosaurus are highly variable and that while those of Protoichthyosaurus were unusual, they did not seem sufficiently different to support the latter's status as a separate genus. They also noted that the traits Appleby used to distinguish Protoichthyosaurus only pertained to foreflipper anatomy and represented the ancestral condition for Ichthyosaurus rather than being novel evolved features. However, Maisch and Matzke noted that P. prostaxalis and P. prosostealis could potentially represent valid species of Ichthyosaurus.

In 2017, Dean Lomax, Judy Massare, and Rashmiben Mistry instead found Protoichthyosaurus to be distinct from Ichthyosaurus, based on features of the skull and shoulder girdle, in addition to those of the foreflipper. Specimens from Dorset, Somerset, Nottinghamshire, Leicestershire, and Warwickshire in England as well as Glamorgan in Wales were considered attributatable to Protoichthyosaurus, making it one of the most widespread Early Jurassic ichthyosaurs of Britain. The authors noted that the variability of foreflipper anatomy in Ichthyosaurus had previously been overestimated. Some of this variability is due to pathologies, such as in the sole specimen of P. prosostealis. Save for an additional flipper bone, this specimen is very similar to Ichthyosaurus and therefore probably an aberrant individual of that genus, rather than a distinct species. Additionally influencing the apparent variability of Ichthyosaurus, many skeletons from historical collections actually represent composites of multiple individuals, and sometimes contain reconstructed parts or were dissassembled then reassembled incorrectly. Many of the original Protoichthyosaurus specimens come from the Charles Moore collection, in which there are multiple composites.

In their review of the genus, Lomax and colleagues found P. prostaxalis to be a valid species, and its type specimen and the additional specimen BRLSI M3555 to each be composed of only a single individual's remains. Further specimens, including the partial skeleton AGC 12 and the skulls BU 5323 and WARMS G347, were also found to be attributable to P. prostaxalis. However, the researchers found BRLSI M3563 to have one foreflipper belonging Protoichthyosaurus associated with a scapula (shoulder blade) and ribs and another foreflipper belonging to Ichthyosaurus, with the rest of the skeleton's affinity being unclear. LEICT G454.1951/164 was discovered to be lost, and may actually be a hindflipper of Ichthyosaurus. (Lomax2017) The specimen UNM.G.2017.1, a largely authentic skeleton except for probably much of its tail, was assigned to Protoichthyosaurus by Lomax and colleagues. However, they noted that there were differences between its skull and humerus (upper arm bone) and those of P. prostaxalis, so they placed it in a new species, P. applebyi, named in honor of Appleby and his work on ichthyosaurs. A possible specimen of this second species, originally thought to be Ichthyosaurus and later reclassified as P. cf. applebyi, was discovered from the lower Sinemurian-aged Staffelegg Formation of Switzerland.

The nominal species Ichthyosaurus fortimanus Owen, 1849-1884 based on the holotype forefin NHMUK R.1063 and synonymized with Ichthyosaurus communis by McGowan (1974), was referred to Protoichthyosaurus, as P. fortimanus by Lomax and Massare in 2018, based on comparisons with known Ichthyosaurus and Protoichthyosaurus species. However, as this foreflipper does not preserve the diagnostic traits of the other two Protoichthyosaurus species, the researchers noted that it could potentially be synonymous with one of them. In the same publication, they also described additional specimens, including a skull assigned to P. applebyi, the second known specimen of that species. In 2019, Lomax, Laura Porro, and Nigel Larkin found that BMT 1955.G35.1, a partial skeleton initially identified as belonging to Ichthyosaurus communis, instead pertained to Protoichthyosaurus prostaxalis. This specimen was considerably larger than previous specimens of P. prostaxalis, and its good preservation allowed a three-dimensional reconstruction of the animal's skull to be created. In 2020, Lomax, Massare, and Mark Evans assigned two partial skulls to P. prostaxalis that revealed further data on the skull roof. They also identified two partial forelimbs as belonging to the genus, but could not assign them to either species with confidence.

Protoichthyosaurus belongs to the group Ichthyosauria, marine reptiles with dolphin-shaped bodies. Like other members of this group, its limbs were modified into flippers and it had a tall tail fin for propulsion. Protoichthyosaurus prostaxilis is a medium-sized ichthyosaur, with the total length of the largest specimen likely somewhere between 3.2–4 metres (10–13 ft), while that of other known specimens likely being below 2.5 metres (8.2 ft). P. applebyi is smaller, probably not exceeding 2 metres (6.6 ft) in total length.