Mithridates I of Parthia or Mithradates (c. 195 BC – 138 BC) was the "Great King" of Parthia from about 171 BC to 138 BC, succeeding his brother Phraates I. He was the son of Phriapatius, King of Parthia (died c. 176 BC). He made Parthia into a major political power, expanded the empire westward into Mesopotamia. During his reign, the Parthians took Babylonia (144 BC), Media (141 BC) and Persia (139 BC), when Mithridates captured the Seleucid ruler Demetrius II of Syria. Demetrius later married Mithridates's daughter Rhodogune.

Eastward Mithridates extended his control into Margiana, Aria and Bactria, thus completing Parthian control of the overland trade routes between east and west, the Silk Road and the Persian Royal Road. This control of trade was to be the foundation of Parthian wealth and power, and was jealously guarded by the Arsacids, who always attempted to maintain direct control over the lands through which the major trade routes passed.

Parthian control broke the tenuous link with Greeks in the West that had sustained the Hellenistic kingdom of Bactria, yet Mithradates actively promoted Hellenism in the areas he controlled and titled himself Philhellene ("friend of the Greeks") on his somewhat primitive coins: they show the first appearance on Parthian coins of a Greek-style portrait showing the royal diadem, the standard Greek symbol for kingship.. Mithradates resumed the striking of coins, which had been suspended ever since Arsaces II (c. 211–191 BC) had been forced to submit to the Seleucid Antiochus III in 206 BC.

His name assigned him to the protection of Mithra and carried the god's authority in some measure.

His son Phraates II succeeded him on his death as King.