For the polychaete worm genus, see Orseis (polychaete).

In Greek mythology, Orseis (; ) was the water-nymph (Naiad) of a spring in Thessalia, Greece, and the mythical ancestor of the Greeks. It is uncertain whether she was believed to be the daughter of Oceanus or the river-god of Thessalia, Peneios. There is even a possibility that she was the daughter of Zeus and Deino the Graeae. According to the Library, Orseis married Hellen, son of Deucalion and Pyrrha and brother of Pandora, the legendary eponymous ancestor of the Greeks. Their sons, Dorus, Xuthus, and Aeolus, according to Hesiod's Eoiae or Catalogue of Women together with the sons of Pandora, Graecus, Magnetas and Makedon with Zeus, became the founders of the seven primordial tribes of Hellas (Graecians, Magnetes, Makedones, Dorians, Achaeans, Ionians, and Aeolians).