Ichthyotitan (/ˌɪkθiəˈtaɪtən/ IK-thee-ə-TY-tən) is an extinct genus of giant ichthyosaur from the Late Triassic (Rhaetian), known from the Westbury Mudstone Formation in Somerset, England.

Estimates scaling up the bones from other ichthyosaur species put Ichthyotitan's body length at nearly 25 metres (82 ft), which would make it the largest marine reptile currently known.

In 2018, Dean Lomax, Paul de la Salle, Judy Massare, and Ramues Gallois identified the Lilstock specimen as belonging to a shastasaurid.

Subsequent expeditions were led by the team, revealing additional pieces until 16 October 2022, and known parts of the surangular, a lower jaw-bone, were reassembled in that same year.

[4] Other fragmentary remains were discovered but not identified with the holotype specimen, including two large rib sections from another potential giant ichthyosaur, found at a higher stratigraphic level.

[3] Other fragmentary remains of giant ichthyosaurs of a similar age to Ichthyotitan have also been reported from Germany (Bonenburg [de] district of Warburg) and France (Autun and Cuers).

Comparing the position of the MAME process in the BAS specimen to that in Besanosaurus, they provided a revised length estimate of 25 metres (82 ft), likely making it the largest marine reptile ever described.

While less well-preserved, the anterior part of the surangular bears a lateral groove believed to represent the continuation of the fossa surangularis, also known from the Cuers specimen.

[3] Similarly, unique patterns of periosteal growth in Ichthyotitan are believed to have played a role in approaching the biological size limits in vertebrates.

This is because their short and narrow hyoid bones are unsuitable to withstand impact forces for such kind of feeding[10] and some species like Shonisaurus had robust sectorial teeth with gut contents of mollusc shells and vertebrates.

This has been taken as evidence for the richness of marine food webs throughout the Triassic, believed to be built upon newly evolved forms of plankton, and would show that shastasaurids were flourishing until their disappearance in the end-Triassic extinction.