Which Byzantine church in Istanbul is commonly called the Little Hagia Sophia?

The Byzantine church commonly called the Little Hagia Sophia in Istanbul is the Church of Saints Sergius and Bacchus.

The church was built in Constantinople during the reign of Emperor Justinian I, probably between 527 and 536. It was dedicated to Saints Sergius and Bacchus, soldiers and Christian martyrs who were especially honored by Justinian and Empress Theodora. Its compact domed design is often viewed as an important predecessor to Hagia Sophia.

The building has an unusual octagonal central space enclosed within a roughly square exterior. This arrangement helped Byzantine architects explore ways to support a large dome while combining centralized and basilican planning. The church’s form is therefore important in the development of Byzantine architecture.

After the Ottoman conquest of Constantinople, the building was converted into a mosque. It is still an active mosque and is often identified by its modern descriptive nickname, Little Hagia Sophia, rather than by its original dedication.