The Little Hagia Sophia mosque (Turkish: Küçük Ayasofya Camii), formerly the Church of Saints Sergius and Bacchus (Ancient Greek: Ἐκκλησία τῶν Ἁγίων Σεργίου καὶ Βάκχου ἐν τοῖς Ὁρμίσδου, romanized: Ekklēsía tôn Hagíōn Sergíou kaì Bákchou en toîs Hormísdou), is a former Greek Orthodox church dedicated to Saints Sergius and Bacchus in Constantinople (modern Istanbul), built between 532 and 536, and converted into a mosque during the Ottoman Empire.

This Byzantine building with a central dome plan was erected in the sixth century by Justinian; despite its Turkish name, it likely was not a model for Hagia Sophia ("Holy Wisdom"), with which its construction was contemporary, but it is nonetheless one of the most important early Byzantine buildings in Istanbul. It was recognized at the time by Procopius 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".