Aegirosaurus is an extinct genus of platypterygiine ophthalmosaurid ichthyosaurs known from the late Jurassic and early Cretaceous of Europe.

It has been referred to the species Ichthyosaurus trigonus posthumus (later reclassified in the dubious genus Macropterygius) in the past, and sometimes identified with Brachypterygius extremus.

In 2000, Bardet and Fernández selected a complete skeleton in a private collection as the neotype for the species I. leptospondylus, as the only other described specimen was destroyed in World War II.

Aegirosaurus lineage was found to include Brachypterygius and Maiaspondylus, too, and to nest within the Platypterygiinae, which is the sister taxon of Ophthalmosaurinae.

Its remains were discovered in the Solnhofen limestone formations, which have yielded numerous well-known fossils, such as Archaeopteryx, Compsognathus, and Pterodactylus.

Ichthyosaurs are marine reptiles showing extreme adaptations to life in the water, superficially resembling fish and dolphins in overall shape.

[1] The skull of Aegirosaurus gently tapers into an elongate, thin snout that makes up 62-73% of the length of the lower jaw.

The external nares (nostril openings) are long, thin, and two-lobed in shape,[8] with one lobe protruding upwards.

[1] The radius is significantly smaller than the ulna, and the intermedium is longer from its lower to upper end than it is wide.

Bardet and Fernández reported three types of skin impressions in the neotype: a wavy texture running parallel to the body; straight, fibrous tissue at a right angle to the ripples; and what appeared to be very small scales.

This last texture was found on the upper lobe of the caudal fin, and would contradict the idea that ichthyosaurs lacked scales.

specimen JME-SOS-08369 by Lene Delsett and colleagues in 2022, however, found it to have smooth, scaleless skin, as typical for ichthyosaurs.

[10] Tooth morphology and wear pattern suggest that Aegirosaurus belonged to the "Pierce II/ Generalist" feeding guild.