Avşa Island or Türkeli is a Turkish island in the southern Sea of Marmara with an area of about . It was the classical and ByzantineAphousia or Ophiousa (Οφιούσα) and was a place of exile during the Byzantine period.
The island belongs to the Marmara District of Balıkesir Province in northwestern Turkey. It is a popular domestic tourist destination, especially for tourists from Istanbul. The local population is around 2,000 according to the last census, but during the summer season the number of visitors increases as far as forty to fifty thousand.