Holdenius is an extinct genus of arthrodire placoderm fish which lived during the Late Devonian period.

Holdenius was a piscivorous animal that used its sharp shearing gnathal plates to seize and cleave its prey into manageable pieces.

One articulated specimen of this placoderm from the Upper Devonian Cleveland Shale was preserved adjacent to the remains of its prey; a Ctenacanth chondrichthyan, which had been bitten in half.

An anterior dorsal spine from the ctenacanth was found lodged in the palate and extending into the braincase of the Holdenius, likely killing it instantly.

Previously classified as belonging to the family Dunkleosteidae, it is now considered to be a member of Aspinothoracidi, the sister clade to Dunkleosteoidea.