Alexander Markowich Ostrowski (Ukrainian: Олександр Маркович Островський; Russian: Алекса́ндр Ма́ркович Остро́вский; 25 September 1893 – 20 November 1986) was a mathematician.

His father Mark having been a merchant, Alexander Ostrowski attended the Kiev College of Commerce, not a high school, and thus had an insufficient qualification to be admitted to university.

During World War I he was interned, but thanks to the intervention of Hensel, the restrictions on his movements were eased somewhat, and he was allowed to use the university library.

After the war ended, Ostrowski moved to Göttingen where he wrote his doctoral dissertation and was influenced by Hilbert, Klein and Landau.

In 1920, after having obtained his doctorate from the University of Göttingen,[1] Ostrowski moved to Hamburg where he worked as Hecke's assistant and finished his habilitation in 1922.