Mount Athos (/ˈæθɒs/; Greek: Άθως [ˈaθos]) is a mountain and peninsula in northeastern Greece and an important centre of Eastern Orthodox monasticism. It is governed as an autonomous polity within the Greek Republic. Mount Athos is home to 20 monasteries under the direct jurisdiction of the Ecumenical Patriarch of Constantinople.

Mount Athos is commonly referred to in Greek as the "Holy Mountain" (Greek: Ἅγιον Ὄρος, Agion Oros [ˈaʝ(i)o(n) ˈoros]) and the entity as the "Athonite State" (Greek: Αθωνική Πολιτεία, Athoniki Politia)[citation needed]. Other languages of Orthodox tradition also use names translating to "Holy Mountain" (e.g. Bulgarian and Serbian Света гора, Russian Святая Гора, Georgian მთაწმინდა). In the classical era, while the mountain was called Athos, the peninsula was known as Acté or Akté (Ἀκτή).

Mount Athos has been inhabited since ancient times and is known for its nearly 1,800-year continuous Christian presence and its long historical monastic traditions, which date back to at least 800 A.D. and the Byzantine era. Today, over 2,000 monks from Greece and many other countries, including Eastern Orthodox countries such as Romania, Moldova, Georgia, Bulgaria, Serbia and Russia, live an ascetic life in Athos, isolated from the rest of the world. The Athonite monasteries feature a rich collection of well-preserved artifacts, rare books, ancient documents, and artworks of immense historical value, and Mount Athos has been listed as a World Heritage Site since 1988.

Although Mount Athos is technically part of the European Union like the rest of Greece, the status of the Monastic State of the Holy Mountain, and the jurisdiction of the Athonite institutions, were expressly described and ratified upon admission of Greece to the European Community (precursor to the EU). The free movement of people and goods in its territory is prohibited, unless formal permission is granted by the Monastic State's authorities, and only males are allowed to enter.