Agathonísi (Greek: Αγαθονήσι; Italian: Gaidaro) is a small Greek island and municipality located at the northernmost point of the Dodecanese in Greece. It is surrounded by many smaller islands and is home to two villages, both inland; Megálo Chorió ("Big Village"), and Mikró Chorió ("Small Village"). The island's only port is the settlement of Agios Georgios (Saint George), which consists of a few hotels. The island is also locally known as Gaidaro, or by its ancient name Tragea.
The highest point on the island is 209 metres above sea level. This peak is located close to Mikro Chorio. The island covers an area of 13.5 km².
A census of the island in 1981 showed that it was populated by 133 people. In 1991, another census showed that the population had dropped to 112. By the 2001 census it had again risen to 158 residents, and in 2011 its population was 185, 168 of whom lived in Megálo Chorió, and only 17 in Mikró Chorió. The municipality of Agathonisi, which includes the uninhabited offshore islets of Gláros, Kounéli, Nerá, and Psathonísio, has a combined land area of 14.500 km².