The Ka'usiyeh Dynasty rules Tabaristan (now Mazandaran in northern Iran) from 665 to about 1006.

It is a branch of the Bavand Dynasty, based in a mountainous region southwest of Sari, whose isolation plays a rôle in the survival of the early dynasty.The dynasty is founded by the founder of the Bvads, Baw Ka'usiyeh, the first prince of Tabaristan (r. 665–680) son of Kawus, son of Kavadh I, the Sasanian Emperor.In about 854, Qaren I (r. 837–867) converts to Islam.

Rostam III (r. 1006–57) becomes a vassal of the Ziyarid king Shams al-Mo'ali Abol-hasan Ghaboos ibn Wushmgir (or Qabus), but as the Ziyarids' power declines, Rostam (and Qaren II after him) becomes an essentially independent minor ruler.In about 1074, another branch of the Bavand Dynasty – the Espahbadiyeh – succeeds the Ka'usiyeh, and rules over Mazandaran and Gilan until 1210.