The Church of Hagia Euphemia at the Hippodrome was a Byzantine church in Constantinople. It was original the Palace of Antiochus, where the influential Persian eunuch Antiochus once lived. It was converted into a church and housed the relics of St. Euphemia in the 7th century. The church has a cycle of fourteen frescoes dating to the late 13th century located in the southwestern section of the church.

The original church dedicated to St. Euphemia was built in the 4th century in Chalcedon. It consisted of a basilica with an attached circular martyrion in which the “uncorrupted body” of St. Euphemia was kept in a silver sarcophagus. The Council of Chalcedon was held here in 451. A later tradition holds that St. Euphemia miraculously determined the outcome of the Ecumenical Council at Chalcedon in 451. The Orthodox and the Monophysites placed their doctrinal formula in her coffin. Later, when the coffin was opened, they miraculously fund the Monophysite creed under Euphemia’s feet and the Orthodox one in her hands. The Persian invasions of the early 7th century caused its destruction, leading to her relics being Constantinople and was housed in the converted great hall of the Palace of Antiochos next to the Hippodrome.After being confiscated, the Palace of Antiochus became an imperial possession serving various functions. The relics of St. Euphemia were transferred to Constantinople from Chalcedon in either 615 or 626 when Persian attacked the Asian shore around Chalcedon. It has been argued that the palace was converted in the 6th century before the relics were transferred, during which time a synthronon was added in the eastern niche. Once it was converted, tombs and mausolea were added around the building. 
The relics of St. Euphemia were supposedly removed during Iconoclasm. According to one legend, Constantine V (741-775) had the relics thrown into the sea, but they were miraculous recovered and brought to the island of Lemnos. Their return to Constantinople was celebrated by Empress Irene (797-804) and her son Constantine VI around 796, after which the church was redecorated. It was probably damaged by a fire in 1203 that destroyed the Hippodrome of Constantinople. The church, having suffered during the Latin occupation of the city, was restored and adorned with frescoes. It seems that the church was destroyed during the construction of the Ibrahim Pasha’s palace in 1522. The relics of St. Euphemia were moved to the Patriarchate, which can be found in the Patriarchal Church of St. George. 

The first remains discovered on the site in 1939 were frescoes on the remains of a wall, which allowed for its identification as the Church of St. Euphemia.  In 1942, a hexagonal hall with a semicircular portico was revealed, while excavations in 1951-52 uncovered a column base with an inscription ‘Of Antiochus the praepositus’. Evidence from brickstamps suggests it was built sometime after 429. 

The original palace consisted of a hexagonal hall with a semicircular (sigma-plan) portico. The hexagonal structure had a diagonal around 20 meters, with an apse on each side of the hexagon, except for the side with the entrance. Small circular rooms were inserted between two neighboring apses. The hexagon was preceded by a wide semicircular portico. A new opening was made at the west niche of the hexagonal structure, while a synthronon and altar were added to the east niche. While literature sources suggest it was first converted into a church in the 7th century, its architecture features suggest it was first converted in the 6th century. Several fragments were discovered on the site, including a chancel barrier, chancel-slab decorations and a column decorated with inlaying glass paste in marble