Orestis (Greek: Ὀρεστίς) was a region of Upper Macedonia, corresponding roughly to the modern Kastoria regional unit located in West Macedonia, Greece. Its inhabitants were the Orestae, an ancient Greek tribe that was part of the Molossian tribal state, or koinon.
The term Orestis is derived from the Greek word orestias meaning "of the mountains" or "mountainous".
Like most of Upper Macedonia, Orestis only became part of Macedon after the early 4th century BC; before that, it had close relations with Epirus. A 6th century BC silver finger ring bearing the frequent Orestian name "Antiochus" was found in the Dodona sanctuary. During the Peloponnesian War, a thousand Orestians led by King Antiochus accompanied the Parauaeans of Epirus. Hecataeus and Strabo identified these mountain kingdoms as being of Epirotic stock. Natives of the region were: Pausanias of Orestis, the lover and murderer of Philip II, and three of Alexander's prominent diadochi: Perdiccas (son of Orontes), Seleucus I Nicator (son of Antiochus) and Craterus, son of a noble from Orestis named Alexander.
The region became independent again in 196 BC, when the Romans, after defeating Philip V (r. 221–179 BC), declared the Orestae free  because they had supported the Roman cause in the recent war against Macedon. According to Appian, Argos Orestikon (in modern Orestida), rather than Peloponnesian Argos, was the homeland of the Argead dynasty.
- Boardman & Hammond 1982, p. 266: "On crossing the Balkan chain, we find that Hecataeus called the Orestae 'a Molossian tribe' (F 107), and Strabo (434; cf. 326) probably derived from Hecataeus his belief that the Elimeotae, Lyncestae, and Pelagones, as well as the Orestae, were Epirotic or rather Molossian tribes before their incorporation by the Macedones into the Macedonian kingdom."
- Hornblower, Spawforth & Eidinow 2012, p. 966: "Molossi, common name of tribes forming a tribal state (koinon) in Epirus, which originated in northern Pindus including the Orestae, FGrH 1 F 107) and expanded southwards, reaching the Ambraciote Gulf (see AMBRACIA) c.370 BC."
- Hammond 1967, p. 703: "The Orestae were Molossian (as we know from a fourth-century inscription)."
- Hammond 2001, p. 158: "Pelagones in the region of Prilep, the Lyncestae in the region of Florina, the Orestae in the region of Kastoria, and the Elimeotae in the region of Kozani. These tribes were all Epirotic tribes and they talked the Greek language but with a different dialect, the Northwest Greek dialect, as we know now from the local questions which were put to the god of Dodona."
- Borza 1992, p. 74: "The western Greek people (with affinities to the Epirotic tribes) in Orestis, Lyncus, and parts of Pelagonia."
- Liddell & Scott 1940: ὀρεστιάς
- PAAH (1929) 122.
- Thucydides. History of the Peloponnesian War, 2.80.
-  D. C. Samsaris, Historical Geography of the Roman province of Macedonia (The Department of Western Macedonia today) (in Greek), Thessaloniki 1989 (Society for Macedonian Studies), p. 64-65. ISBN 960-7265-01-7
- Titus Livy. Ab Urbe Condita, 33.34.5: "To the Orestae — that is a tribe of the Macedonians — their own laws were restored, because they had been the first to revolt against the king."
- Appian. Syrian Wars, 11.10.63.
- Boardman, John; Hammond, Nicholas Geoffrey Lemprière (1982). The Cambridge Ancient History - The Expansion of the Greek World, Eighth to Sixth Centuries B.C., Part 3: Volume 3 (Second Edition). Cambridge, UK: Cambridge University Press. ISBN 0-521-23447-6.
- Borza, Eugene N. (1992). In the Shadow of Olympus: The Emergence of Macedon (Revised Edition). Princeton, NJ: Princeton University Press. ISBN 0-691-00880-9.
- Hammond, Nicholas Geoffrey Lemprière (1967). Epirus: The Geography, the Ancient Remains, the History and the Topography of Epirus and Adjacent Areas. Oxford, UK: The Clarendon Press.
- Hammond, Nicholas Geoffrey Lemprière (2001). Collected Studies: Further Studies on Various Topics. V. Amsterdam: Hakkert.
- Hornblower, Simon; Spawforth, Antony; Eidinow, Esther (2012) . The Oxford Classical Dictionary (4th ed.). Oxford: Oxford University Press. ISBN 978-0-19-954556-8.
- Liddell, Henry George; Scott, Robert (1940). A Greek-English Lexicon. Oxford: Clarendon Press.