Oecusse, also known as Oecusse-Ambeno (Portuguese: Oé-Cusse Ambeno; Tetum: Oecussi Ambeno) and formerly just Ambeno, is an exclave, municipality (formerly a district) and the only Special Administrative Region (SAR) of Timor-Leste.

Located on the north coast of the western portion of Timor, Oecusse is separated from the rest of Timor-Leste by West Timor, Indonesia, which is part of the province of East Nusa Tenggara. West Timor surrounds Oecusse on all sides except the north, where the exclave faces the Savu Sea.

The capital of Oecusse is Pante Macassar, also called Oecussi Town, or formerly, in Portuguese Timor, Vila Taveiro. Originally Ambeno was the name of the former district and Oecussi its capital.

Oecusse is the traditional name of Pante Macassar, the present-day capital of the special administrative region, and its environs. Pante Macassar's location was also the seat of the second traditional kingdom of the area, based at Oesono [de]. Ambeno was the historic Timorese empire that occupied most of the present-day SAR's territory; it was centred in Tulaica and Nunuhenu [de].

The portmanteau "Oe-Kussi", from which "Oecusse" is derived, has its origins in the local Baikeno variant of the Uab Meto language. "Oe" means "water", but there are differing interpretations for "Kussi". That word is often equated with the name of a certain type of traditional clay jug, so "Oe-Kussi" would mean something like "water pot". There is a legend about the provenance of this name, relating to a stone, the "Cussi [de]", in the suco of Banafi [de]. Other sources state that Kussi was a native ruler of Ambeno. The name "Ambeno" is derived from "Ambenu", which is similarly a portmanteau of two words. "Ama" or "am" means "father" or "king", and "Benu" is the name of two legendary rulers of the region.

In the Portuguese colonial period, "Oecusse" and "Ambeno" were both already being used as synonyms referring to the exclave. Later, the double-barrelled name Oecusse-Ambeno came into use. In the official list of all administrative divisions of Timor-Leste published in 2009, the then district was referred to only by its short name "Oecusse". The present day SAR is not politically divided along the borders of the old empires of the area. In Ministerial Diploma 16/2017, "Oe-Cusse Ambeno" was once again officially used.

As is not uncommon in Timor-Leste, there are numerous different spellings for the region's name: Oe-Kusi, Oecusse, Ocussi, Oecússi, Oecussi, Oekussi, Oekusi, Okusi, Oé-Cusse. The spellings with "k" are mostly derived from Tetum or other Austronesian languages, and those with "c" are spellings based on Portuguese. As has just been indicated, the double-barrelled name Oecusse-Ambeno (also Oecussi-Ambeno, Ocussi-Ambeno, Oecússi-Ambeno, Oe-Kusi Ambenu) was reactivated in official usage in 2017, in place of Oecusse on its own. The exclave is seldom now referred to only as Ambeno (Ambenu), as it was during the Indonesian occupation of East Timor.

Oecusse SAR has a total area of 813.6 km² (314.1 sq mi). It is completely surrounded by Indonesian territory, except to the north, where it faces Ombai Strait (at that point, part of the Savu Sea). Geographically, the rest of Timor-Leste is 58 km (36 mi) east of the SAR's easternmost tip; by road, the distance is more than 70 km (43 mi).