Fenari Isa Mosque(the Church of St. John the Baptist), is a mosque in Istanbul, made of two former Eastern Orthodox churches.

Location
The complex is located in the Fatih district of Istanbul, Turkey, along the Vatan Caddesi Avenue, in a modern context.

Coloured stone inlay on marble depicting Saint Eudocia, 10./11. century, previously in the church, now in Istanbul Archaeological Museum.In 908, the Byzantine admiral Constantine Lips inaugurated a nunnery in the presence of the Emperor Leo VI the Wise. The nunnery was dedicated to the Virgin Theotokos ("Immaculate Mother of God") in a place called "Merdosangaris" (Greek: Μερδοσαγγάρης), in the valley of the Lycus (the river of Constantinople).

This establishment, which according to its Typicon hosted a total of 50 womenalso a Xenon ("hospital") with 15 beds attached, was known also after his name (Monē tou Livos), and became one of the largest of Constantinople. The church was built on the remains of another shrine from the sixth century, and used the tombstones of an ancient Roman cemetery. Relics of Saint Irene were stored here. The church was generally known as "North Church".

Byzantine remains from the North Church (kept in the Istanbul Archaeological Museums).After the Latin invasion and the restoration of the Byzantine Empire, between 1286 and 1304, Empress Theodora, widow of Emperor Michael VIII Palaiologos, erected another church dedicated to St. John the Baptist (Eκκλησία του Αγίου Ιωάννου Προδρόμου του Λίβος) south of the first church. Several exponents of the imperial dynasty of the Palaiologos were buried there besides Theodora: her son Konstantinos, Empress Eirene of Montferrat and her husband Emperor Andronikos II. This church is generally known as the "South Church".

During the fourteenth century an esonarthex and a parekklesion were added to the church. The custom of burying members of the imperial family in the complex continued in the fifteenth century with Anna, first wife of Emperor John VIII Palaiologos, in 1417.The church was possibly used as a cemetery also after 1453.