Blachernae (Greek: Βλαχέρναι) was a suburb in the northwestern section of Constantinople. It was the site of a spring and a number of prominent churches were built there, most notably the great Church of St. Mary of Blachernae (Panagia Blacherniotissa), built by Empress Pulcheria in ca. 450, expanded by Leo I and renovated by Emperor Justinian I in the 6th century.
The quarter is recorded as regio XIV in the early 5th-century Notitia Urbis Constantinopolitanae, where it is recorded as being enclosed by a wall of its own. The quarter was connected to the city proper at the construction of the Theodosian Walls, but the Church of St. Mary remained outside of the walls until 627, when Emperor Heraclius built another wall to enclose it. By that time, the church had become the major Marian shrine of the city, and the second-most important church in Constantinople after Hagia Sophia, if only because the emperors' residence was nearby. In 1347, John VI Kantakouzenos was crowned there, instead of the Hagia Sophia.
South of the church and situated on the city's Seventh Hill stood the imperial Palace of Blachernae, which was first erected in ca. 500. During the Komnenian period, it became the favourite imperial residence, eclipsing the older Great palace on the eastern end of the city. Although the Latin emperors returned to the Bucoleon Palace, the Palaiologos emperors of the restored Byzantine Empire again used the Blachernae Palace as their main residence. The Palace of the Porphyrogenitus (Turkish: Tekfur Sarayı) and the so-called Prison of Anemas are the main surviving structures of the Palace of Blachernae, which was a complex of multiple buildings.
Following the fall of Constantinople to the Ottomans in 1453, the Sultan's residence was moved to Topkapı Palace on the site of the ancient acropolis of Byzantium, opposite to the original site of the Great Palace, which had by this time fallen into complete ruin, and the Blachernae area (with the exception of the Palace of Porphyrogenitus) fell into disuse.
The historic Blachernae area is in the present-day Istanbul quarter known as Ayvansaray. The sacred spring, associated with the Virgin Mary, can still be visited today; in Turkish it is named Ayazma, a name derived from Greek ἁγίασμα hagiasma, meaning "holy place".