Ariarathes IX Eusebes Philopator (Ancient Greek: Ἀριαράθης Εὐσεβής Φιλοπάτωρ, Ariaráthēs Eusebḗs Philopátōr; reigned c. 100–85 BC), was made king of Cappadocia by his father Mithridates VI of Pontus after the assassination of Ariarathes VII of Cappadocia in c. 100 BC.

After a short period of Pontic rule, the Roman Senate intervened and forced him to return the throne to Ariarathes VIII of Cappadocia, after a brief restoration and an attempt of creation of a republic, the Roman Senate directed the Cappadocians to vote by who they wanted to be ruled, and thus the kingdom passed to Ariobarzanes I Philoromaios.

[3] However, as soon as Tigranes II of Armenia and his army returned home, Ariarathes IX of Cappadocia was deposed again by order of the Senate.

[1] He remained king of Cappadocia during the war but was forced to abdicate after his father Mithridates VI of Pontus was defeated by the Romans in 85 BC.

[4] According to Plutarch, Ariarathes IX was poisoned by his father (presumably while serving as a commander of his father's troops in northern Greece during the First Mithridatic War): In the fortress of Caenum, Pompey found also private documents belonging to Mithridates, and read them with no little satisfaction, since they shed much light upon the king's character.