Defterdar Kos Mosque, mosque on Eleftherias Square, Kos, Greece

The Defterdar Kos Mosque is an Ottoman-era house of prayer located on Eleftherias Square in Kos Town. The structure displays a two-story cubic form topped with an octagonal dome supported by twelve arches that frame a square prayer hall inside.

Ibrahim Efendi, an Ottoman defterdar (minister of finance), ordered the mosque's construction at the close of the 18th century; it is named after him. It has been suggested that an Orthodox church dedicated to Saint Paraskevi previously stood on the site. The building was severely damaged by the earthquakes of 1926 and 1933, and the Italians, then holding Kos, carried out restoration work that included removing and restoring the upper part of the minaret.

The mosque incorporates a wooden gynakonitis for women's quarters on the north side of the main hall, while the mihrab and minber stand to the south.

The mosque stands on Eleftherias Square, about 120 metres from the Mandraki harbour of Kos Town, and is reached on foot. Behind it to the east are the Gate of Taxes, now an entrance to the Agora excavation, and a round bastion of the medieval town wall. The building belongs to the Muslim community of Kos.

The mosque stands noticeably askew to the main axes of Eleftherias Square and the surrounding buildings, which were laid out by the Italians after the 1933 earthquake. Behind it, the Gate of Taxes now leads toward the Agora excavation, and a round southeast bastion of the medieval town wall survives nearby.