Homostiidae (alternatively spelled Homosteidae) is a family of flattened arthrodire placoderms from the Early to Middle Devonian.

[1]: 69 The family Homostiidae is considered the basal-most grouping within the suborder Brachythoraci, basal to the large sub-clade Eubrachythoraci, which includes the well-known Dunkleosteus, Dinichthys, etc.

Homostiidae's placement within Brachythoraci can be shown in the cladogram below:[6] Homostius sulcatus Antineosteus lehmani Buchanosteus confertituberculatus Goodradigbeeon australianum Parabuchanosteus murrumbidgeeni Dhanguura johnstoni Gemuendenaspis angusta Xiangshuiosteus wui Coccosteidae Panxiosteidae Incisoscutoidea Rhachiosteus pterygiatus Dunkleosteoidea Aspinothoracidi A comparatively large animal from the Middle Devonian of Siberia, with a head shield estimated around 40 centimeters long.

A very large, primitive form from the Early Devonian Taemas-Wee Jasper Reef fauna.

Tityosteus is thought to be the largest vertebrate known from the Lower Devonian, with an estimated length of 2.5 meters.