Lasanius is a genus of basal jawless fish from the Early Silurian, around 443.8 million years ago, known from fossils found near Lesmahagow, Scotland.

[1] Lasianus has a pair of eyes, and a notochord, and while it has rows of bony scutes running along its back, it lacks the armour typical of other Paleozoic jawless fish like ostracoderms.

There are also structures close to the front of the animal dubbed "chains" and "rods", of unclear function.

At the end of the body a hypocercal tail fin was present in the form of a caudal lobe.

A comprehensive redescription published in 2023 found that it was a stem-group cyclostome, more closely related to hagfish and lampreys than to jawed fish.