Islambol (اسلامبول, Full of Islam) or Islambul (find Islam) or Islam(b)ol (old Turkic: be Islam), both in the Turkish language, were folk-etymological adaptations of Istanbul created after the Ottoman conquest of 1453 to express the city's new role as the capital of the Islamic Ottoman Empire. It is first attested shortly after the conquest, and its invention was ascribed by some contemporary writers to Sultan Mehmed II himself.[4] Some Ottoman sources of the 17th century, most notably Evliya Çelebi, describe it as the common Turkish name of the time. Between the late 17th and late 18th centuries, it was also in official use. The first use of the word "Islambol" on coinage took place in 1703 (1115AH) during the reign of Sultan Ahmed III. The term Kostantiniyye still appeared, however, into the 20th century.

Names other than استانبول (İstanbul) had become obsolete in the Turkish language during the late Ottoman/early republican periods. However, Constantinople was still used when writing the city's name in Latin script. In 1928, the Turkish alphabet was changed from Arabic script to Latin script. After that, Turkey started to urge other countries to use Turkish names for Turkish cities, instead of other transliterations to Latin script that had been used in the Ottoman times.[20] Letters or packages sent to "Constantinople" instead of "Istanbul" were no longer delivered by Turkey's PTT, which contributed to the eventual worldwide adoption of the new name.