The Epistula Mithridatis (Latin: "Letter of Mithridates"), also known as the Letter of Mithridates to King Arsaces, is a letter allegedly written by Mithridates VI of Pontus to the Parthian king Phraates III (r. 70–57 BC). The letter was discovered "among a collection of Sallustian speeches and letters probably originally produced in the first or second centuries A.D". In the letter, Mithridates requests Parthian aid against the troops of Lucullus, and in a series of arguments, Mithridates asks the Parthians to join an alliance with Mithridates and Tigranes (king of Armenia), against the Romans. The letter, assigned to Sallust, is considered to be an important source on the Pontic–Parthian relations at the time.