Onomarchus (Greek: Ονόμαρχος) was general of the Phocians in the Third Sacred War, brother of Philomelus and son of Theotimus.Onomarchus commanded a division of the Phocian army under Philomelus in the action at Tithorea, in which Philomelus perished. After the battle Onomarchus gathered the remains of the Phocian army and retreated to Delphi.