The Defterdar Mosque, or the Defterdar Mahmut Efendi Mosque (Turkish: Defterdar Camii, Defterdar Mahmut Efendi Camii), is 16th century Ottoman mosque located in Eyüp, Istanbul, Turkey. It was commissioned by defterdar (chief finance secretary) Nazlı Mahmut Efendi (c. 1500–1546) and built by architect Mimar Sinan in 1542. Instead of a crescent, this mosque has "ink pot and pen" on top of its dome, representing the profession of the founder of the mosque (defterdar derives from defter, 'notebook, register', and the suffix -dar, 'doer'). The original pair was broken by a storm in 1997. Ten years later, on 30 May 2007, a new inkpot and a pen assembled on top of the dome of the mosque.