Penthilus /ˈpɛnθᵻləs/ (Greek: Πένθιλος) is the name for two distinct people in Greek mythology.
Son of Orestes and Erigone
Penthilus was the illegitimate or legitimate son of half-siblings Orestes and Erigone. Penthilus' grandmother was Clytemnestra. His maternal and paternal grandfathers were Aigisthos and Agamemnon respectively. Orestes killed both Clytamnestra, who was his own mother and Aigisthos. Penthilus' mother is said to have hanged herself or married Penthilus' father after Penthilus' father's wife, Hermione died. His father was ruler over much of the Peloponnese and died of a snakebite at age 70. One story says that as a child, Penthilus was torn apart and devoured by wolves in the Taygetus mountains, near Sparta. His father established a festival of mourning, the so-called Penthilia in his honour.
According to Pausanias, Penthilus grew up and founded a city either on Lesbos or in Thrace. His grandson Gras founded a city Aeolis, between Ionia and Mysia.
Penthilus had a son named Damasias who fathered Gras.
King of Messenia
Penthilus was also the name of a king of Messenia, son of Periclymenus and father of Borus.