Istanbul University (Turkish: İstanbul Üniversitesi), also known as University of Istanbul, is a public research university located in Istanbul, Turkey. Founded by Mehmed II on May 30, 1453, a day after the Ottoman conquest of Constantinople, it was reformed as the first Ottoman higher education institution influenced by European approaches. The successor institution, which has been operating under its current name since 1933, is the first university in modern Turkey.

The university has 58,809 undergraduate, graduate, and doctoral students studying in 112 academic units, including faculties, institutes, colleges, and vocational schools at 9 campuses. The main campus is adjacent to Beyazıt Square in Fatih, the capital district of the province, on the European side of the city.

Istanbul University alumni include Nobel Prize in Chemistry winner Aziz Sancar and Nobel Prize in Literature winner Orhan Pamuk, as well as ex President of Turkey Abdullah Gül, six Prime Ministers of Turkey, Grand Mufti for Bosnia and Herzegovina Mehmed Džemaludin Čaušević, including Suat Hayri Ürgüplü, Sadi Irmak, Nihat Erim, Refik Saydam, Naim Talu, and Yıldırım Akbulut. The alumni also include the longest-serving President of Israel Yitzhak Ben-Zvi, as well as David Ben-Gurion and Moshe Sharett, who both served as Prime Ministers of Israel.

The origins of Istanbul University date back to 1453, when it was founded by Ottoman Sultan Mehmed II as a school of philosophy, medicine, law, mathematics and letters. The University of Constantinople, established in 425 CE by the Eastern Roman emperor Theodosius II as the Pandidacterium, later became known as the Phanar Greek Orthodox College after the Fall of Constantinople in 1453. Immediately after the conquest of the city, Sultan Mehmet II assigned Molla Mehmet Zeyrek Efendi to convert the Pantokrator Monastery into a madrasah and appointed him as the first rector. Madrasah education continued to be given here until new educational buildings were built around the Fatih Mosque. Later, upon completion of the construction of the buildings, students and teachers moved to these new buildings. In the early stages, Istanbul University also functioned as a medrese, an Islamic theological school. Famous astronomer Ali Qushji took part in the establishment of higher education in the city and taught mathematics at the madrasah. This madrasa is regarded as the precursor to the Darülfünûn, a technical institute or university (lit. House of Natural Sciences), which evolved into Istanbul University in 1933. Education in various sciences and fields, including medicine, mathematics, astronomy, cartography, geography, history, philosophy, religion, literature, philology, and law, became available and, until the 19th century, played a key role in educating the ruling cadres of Ottoman society. However, when the medreses were no longer able to meet the needs of the time, a restructuring process began, leading to the establishment of Darülfünûn, the core of Istanbul University.

An institution of higher education named the Darülfünûn-u Osmanî (lit. the Ottoman Polytechnic Institute) was created in 1863, but suppressed in 1871. Its first rector was Hasan Tahsini, regarded as one of the most important Ottoman scholars of the 19th century. In 1874 the Darülfünûn-u Sultanî (lit. the Royal College of Natural Sciences, French: Université Impériale Ottoman) began offering law classes in French, but was closed in 1881.

The Imperial College of Sciences, later known as Darülfünûn-u Şahâne, was reestablished in 1900 with departments in theology, arts, mathematics, science, and philology. In 1924, the faculties of law, medicine, arts and sciences were established in İstanbul Darülfünûnu, as the university was now called. Islamic theology was added in 1925, but in 1933 the university was reorganized without the latter.

The first modern applied physics courses were offered at the Darülfünûn on 31 December 1863, marking the beginning of a new era. On 20 February 1870, the school was renamed Darülfünûn-u Osmanî (lit. the Ottoman House of Multiple Sciences) and reorganized to meet the needs of modern sciences and technologies. Starting in 1874, some classes in literature, law, and applied sciences were held at the Lycée de Galatasaray, continuing regularly until 1881. On 1 September 1900, the school was renamed and reorganized as Darülfünûn-u Şahâne (lit. the Royal Polytechnic Institute), offering courses in mathematics, literature, and theology. On 20 April 1912, the school was renamed İstanbul Darülfünûnu, with an expanded course offering and a modernized curriculum. The School of Medicine, Law, Applied Sciences, Literature, and Theology were established.

On 21 April 1924, the Republic of Turkey recognized İstanbul Darülfünûnu as a state institution. On 7 October 1925, its administrative autonomy was established, and the schools, which had been part of the old medrese system, became modern faculties. The Darülfünûn then consisted of five faculties: medicine, law, letters, theology, and science. The professors were granted academic freedom as outlined in Article 2 of Law 493. On 1 August 1933, İstanbul Darülfünûnu was reorganized as İstanbul Üniversitesi (lit. Istanbul University) following the educational reforms of Mustafa Kemal Atatürk. Classes officially began on 1 November 1933.