Büyükada (Greek: Πρίγκηπος or Πρίγκιπος, rendered Prinkipos or Prinkipo), meaning "Big Island" in Turkish, is the largest of the Princes' Islands in the Sea of Marmara, near Istanbul, with an area of about 2 square miles (5 square kilometres). It is made up of the Maden and Nizam neighbourhoods in the Adalar (Islands) district of Istanbul Province, Turkey.

During the first half of the 20th century, the island was popular with prosperous Greeks and Armenians as a refuge from the summer heat of Istanbul. Nowadays the island's demographics are more similar to a typical suburb of mainland Istanbul.

Historically, many residents of Büyükada were fishermen. However, by the late 2010s tourism to Büyükada swelled enormously as it became a favourite day-trip destination for visitors from greenery-starved Arab countries in particular. The surge in tourism was a major factor in bringing to an end the tradition of using phaetons as the only transport on the island in 2020.

Visitors have been writing about Büyükada since the Turkish travel writer Evliya Çelebi recorded in his Seyahatname (Book of Travels) that there were 200 Greek houses on the island in 1640 and that it was ringed with dalyan fishermen. In 1884 the French historian Gustave Schlumberger published Les Iles des Princes, describing his visit to the archipelago. Ernest Mamboury recorded the sites of the island in his Les Iles des Princes, published in 1943 and Jak Deleon updated his work in 2003 in his Büyükada: A Guide to the Monuments. In 1929 Leon Trotsky settled on the island after being exiled from the Soviet Union; after leaving the island in 1933, he wrote an essay called Farewell to Prinkipo. In 1997 Çelik Gülersoy, who had worked to restore some of the island's buildings, published Büyükada Dün (Büyükada Yesterday). In 2007 John Freely's The Princes' Islands exhaustively listed the historic mansions on the island. In 2009 the poet and translator Joachim Sartorius published an exquisite short travelogue called The Princes' Islands: Istanbul's Archipelago which mainly focused on Büyükada. In 2023, Yapı Kredi Yayınları published Büyükada – The Moris Danon Collection by Büke Uras, which documents a period from the second quarter of the 19th century to the middle of 20th century with photographs and supporting text.

One of the most famous residents who was featured in a Netflix movie of his life was Lefter Küçükandonyadis, a Turkish professional footballer of Greek descent. He is recognised as one of the greatest strikers to play for Fenerbahçe and Turkey.

The island is accessible by Şehir Hatları ferries from Eminönü and Kabataş on the European side of Istanbul and from Kadıköy and Bostancı on the Asian side of the city.

Büyükada is 4.3 km (2.6 miles) long and 1.3 km (0.8 miles) wide. The centre of the island is dominated by two peaks. The one nearest to the ferry landing is the Hill of Jesus (Turkish: İsa Tepesi), which is 164 m (538 ft) high. The second is the Great Hill (Turkish: Yücetepe) which is 202 m (663 ft) high. The island has several small strips of sand and pebble beach too, the most popular being Yörük Ali Plajı near Dilburnu.

Most development on the island is on its northern side with the south still largely wooded.