Sivriada (Greek: Ὀξεία, Oχeia) is one of the Princes' Islands in the Sea of Marmara, near Istanbul.
The island, which has an area of 0.05 km², is officially a neighbourhood in the Adalar district of Istanbul, Turkey.
Sivriada was often used by the Byzantine clerics as a distant place for peaceful worship, and by the Byzantine emperors as a convenient prison to detain prominent people whom they deemed troublesome. The first famous person to be imprisoned in the island by the order of emperor Nikephoros I was Plato of Sakkoudion, the uncle of renowned cleric Theodoros Stoudites, for supporting his nephew in his conflict with the emperor. Other famous people who stayed in the island for religious and political reasons were Gebon, Basil Skleros, Nikephoritzes (the chief minister of Michael VII Doukas), Patriarch John of Constantinople and Patriarch Michael II of Constantinople. The graves of those who died in the island during the Byzantine period can still be seen today.
The ruins of a Roman settlement and a 9th century Byzantine monastery can still be seen on the shore, close to the fishermen's shelter, a small wharf which is often used by yachts. The most important buildings on the island were built in the 9th century AD, including a church, a chapel dedicated to religious martyrs, a monastery on the eastern end (with its walls still seen today) and a cistern in the center of the island (a part of which can still be seen).