Heybeliada or Heybeli Ada (Greek: Χάλκη, Halki) is the second largest of the Princes' Islands in the Sea of Marmara, near Istanbul. It is officially a neighborhood in the Adalar district of Istanbul, Turkey.
The large Naval Cadet School overlooks the jetty to the left as you get off the ferry or seabus. There are two interesting pieces of architecture on the grounds of the school. One is Kamariotissa, the only remaining Byzantine church on the island, and more importantly the last church to be built before the conquest of Constantinople. The other is the grave of Edward Barton, the second English Ambassador to be sent to Constantinople by Elizabeth I of England, who spent his last days in Heybeli in order to escape the plague raging through the city in 1598. His remains were later relocated to the British Cemetery in the Haydarpaşa quarter of the Üsküdar district.
To the right of the jetty lies the town with its bars and cafés, a hotel that stays open all year round, and many lovely Ottoman era wooden houses.
At the top of the central mountain is an 11th-century Greek Orthodox monastery which houses the currently defunct Halki seminary, the main Greek Orthodox seminary in Turkey and Theological Seminary of the Ecumenical Patriarchate of Constantinople. In 1971, parts of the Private University Law were ruled unconstitutional by the Constitutional Court of Turkey; subsequently all private institutions of higher education either became part of the state universities or closed down. Halki's Board of Trustees refused to have it become a part of the University of Istanbul. Consequently, the seminary section of the Halki Theological School was closed down. The high school section is still open, but no longer has students. The monastery attracts tourists from all over Greece and Turkey.