Little Hagia Sophia (Turkish: Küçük Ayasofya Camii), formerly the Church of the Saints Sergius and Bacchus, is a former Eastern Orthodox church dedicated to Saints Sergius and Bacchus in Constantinople, converted into a mosque during the Ottoman Empire.

This Byzantine building with a central dome plan was erected in the sixth century by Justinian, likely was a model for Hagia Sophia (St. Sophia), and is one of the most important early Byzantine buildings in Istanbul. It was recognized at the time as an adornment to the entire city, and a modern historian of the East Roman Empire has written that the church, “by the originality of its architecture and the sumptuousness of its carved decoration, ranks in Constantinople second only to St. Sophia itself”.

Little Hagia Sofia Mosque / St. Sergius and Bacchus Church is located between Cankurtaran and Kadırga quarters in Eminönü District, 20 km away from the southern seaside of Mediterranean ramparts. Although it is stated in some sources that there was a pavilion of Big Palace, which is known as Hormidas Palace, and a basically planned church established for Apostle Petrous and Pavlos near Small St. Sofia Mosque, there is no proof which determines their exact locations.

Small St. Sofia Mosque or St. Sergius and Bacchus church with its former name, which is the useable oldest structure of İstanbul today, was constructed between the years 527-536. According to the legends stated in the sources about the construction of the building (Millingen 1912), at the 1st Anastasyus Period, 1st Justiniaunus and his uncle 1st Justinos were condemned to death due to an allegation that they had a rebellion against the Emperor Anastasyus. One night before the execution, the Emperor Anastasyus sees the saints St. Sergius and Bacchus in his dream and the saints testifies in favour of 1st Justiniaunus and 1st Justinos. The emperor, who is affected by this dream, forgives them. When 1st Justiniaunus becomes emperor, he establishes St. Sergius and Bacchus church as a vow church in order to show his gratitude to these saints.

After the conquer of İstanbul, the building, which was used as a church for nearly 1000 years, was changed to a mosque by Hüseyin Agha, the Kapu Agha, in 1504 during the 2nd Bayezid period.