Crassigyrinus was an early tetrapod from the early Carboniferous period of Scotland.

Crassigyrinus had a streamlined body up to 2 metres (6.6 ft) in length. Its limbs were tiny and virtually useless, implying that the animal was almost completely aquatic. Crassigyrinus had unusually large jaws, equipped with two rows of sharp teeth, the second row having a pair of palatal fangs. These would have been ideal for catching fish, and the animal was probably a fast-moving predator. Crassigyrinus had large eyes, suggesting that it was either nocturnal, or lived in very murky water. It possessed large otic (spiracular) notches, probably accommodating a spiracle rather than a tympanic membrane.

Some scientists regard Crassigyrinus as a reptiliomorph.