Mesacanthus ('middle spine') is an extinct genus of acanthodian fish from Devonian Scotland.

[1] Overall the genus is small (average length = 30mm) and fairly conservative, anatomically speaking, for acanthodians.

[1][2] The genus was erected by Ramsay Traquair in 1888 to accommodate certain species that had been previously assigned to Acanthodes by Louis Agassiz.

[3] The genus is found in both Lower Old Red Sandstone and Middle Old Red Sandstone assemblages, with M. pusillus, M. peachi and M. coriaceus known from the Middle Devonian and M. mitchelli being the only known species from the Lower Devonian.

[4] In 2015 a study published in PeerJ reassessed the two remaining Middle Devonian species (M. pusillus and M. peachi) and found that they also could not be distinguished from one another.