In Greek mythology, Polymestor or Polymnestor (Ancient Greek: Πολυμ(ν)ήστωρ) was a King of Thrace. His wife was Ilione, the eldest daughter of King Priam. Polymestor appears in Euripides' play Hecuba and in the Ovidian myth "Hecuba, Polyxena and Polydorus".
During the Trojan War, King Priam was frightened for his youngest son Polydorus's safety since Polydorus could not fight for himself. Priam sent the child, along with gifts of jewelry and gold, to the court of King Polymestor to keep him away from the fighting. After Troy fell, Polymestor betrayed Priam and threw Polydorus into the ocean in order to keep the treasure for himself.
Hecuba, Polydorus' mother, found the body and discovered the treachery. She asked Agamemnon to bring Polymestor to her. Agamemnon complied, motivated by the love of Cassandra, another of Hecuba's children. Hecuba baits Polymestor by drawing him in with treasure. Hecuba has the other Trojan women kill Polymestor's sons, and blinds Polymestor by scratching his eyes out. Polymestor is humiliated at having been blinded and made childless at the hands of slave women. Polymestor is given a trial against Hecuba by Agamemnon. Polymestor claims to be working in the Greek's interest by killing Polydorus before he avenges his brothers and father. Hecuba refutes this claim by stating that Greece has no interest in allying with barbarians. Agamemnon sides with Hecuba and declares Polymestor's actions to be murder. Agamemnon has his soldiers seize Polymestor. As he is being taken away, Polymestor reveals the deaths of Hecuba and Agamemnon.