Stefan Mazurkiewicz
Stefan Mazurkiewicz 1935
Born	(1888-09-25)25 September 1888 Warsaw, Congress Poland, Russian Empire
Died	19 June 1945(1945-06-19) (aged 56) Grodzisk Mazowiecki, Poland
Nationality	Polish
Alma mater	University of Lwów
Known for	Hahn–Mazurkiewicz theorem
Scientific career
Fields	Mathematics
Institutions	University of Warsaw
Doctoral advisor	Wacław Sierpiński

Stefan Mazurkiewicz (25 September 1888 – 19 June 1945) was a Polish mathematician who worked in mathematical analysis, topology, and probability. He was a student of Wacław Sierpiński and a member of the Polish Academy of Learning (PAU). His students included Karol Borsuk, Bronisław Knaster, Kazimierz Kuratowski, Stanisław Saks, and Antoni Zygmund. For a time Mazurkiewicz was a professor at the University of Paris; however, he spent most of his career as a professor at the University of Warsaw.

The Hahn–Mazurkiewicz theorem, a basic result on curves prompted by the phenomenon of space-filling curves, is named for Mazurkiewicz and Hans Hahn. His 1935 paper Sur l'existence des continus indécomposables is generally considered the most elegant piece of work in point-set topology.

During the Polish–Soviet War (1919–21), Mazurkiewicz as early as 1919 broke the most common Russian cipher for the Polish General Staff's cryptological agency. Thanks to this, orders issued by Soviet commander Mikhail Tukhachevsky's staff were known to Polish Army leaders. This contributed substantially, perhaps decisively, to Polish victory at the critical Battle of Warsaw and possibly to Poland's survival as an independent country.