Aleksander "Olek" Pelczynski (2 July 1932, Tarnopol, Poland – 20 December 2012, Wroclaw) was a Polish mathematician.
Pelczynski studied mathematics from 1950 to 1956 at the University of Warsaw and received there his doctorate in 1958 under Stanislaw Mazur with dissertation Wlasnosci izomorficzne przestrzeni Banacha zwiazane ze slaba zbieznoscia bezwarunkowa szeregów (Isomorphic properties of Banach spaces with regard to unconditional convergence of series).
From 1967 to 2002 he worked at the Polish Academy of Sciences.
Since 1967 he was a member of the editorial staff of the journal Studia Mathematica.
Pelczynski's main field of research was functional analysis, especially the theory of Banach spaces.
In 1961 he received the Stefan Banach Prize and in 1996 the Stefan Banach Medal of the Polish Academy of Sciences.
In 2005 he received an honorary doctorate from the Adam Mickiewicz University in Poznan.
The Bessaga-Pelczynski Selection Principle and the Pelczynsk Decomposition Method are associatedn with his name.
In 1966 in Moscow he was (with Boris Mityagin) an Invited Speaker of the ICM.
In 1983 in Warsaw Pelczynski was a Plenary Speaker of the ICM (Structural Theory of Banach Spaces and Its Interplay with Analysis and Probability).
In 1986 he was elected a member of the Akademie der Wissenschaften der DDR.
His doctoral students include Nicole Tomczak-Jaegermann and Stanislaw Szarek.
He died in December 2012 and was buried in Warsaw.