Onceium or Onkeion (Ancient Greek: Ὄγκειον) was a village in ancient Arcadia upon the river Ladon, near Thelpusa, and containing a temple of Demeter Erinnys.[1][2] The Ladon, after leaving this temple, passed that of Apollo Oncaeates on the left, and that of the boy Asclepius on the right.[3] The name is derived by Pausanias from Oncus, a son of Apollo, who reigned at this place. Other writers mention a small town Oncae (Ὄγκαι) in Arcadia, which may be the same as Onceium.[4]
Its site is unlocated.[5]