Alexander Aigner (18 May 1909 – 2 September 1988) was number theorist and a full university professor for mathematics at the Karl Franzens University in Graz, Austria. During World War II he was part of a group of five mathematicians, which was recruited by the military cryptanalyst Wilhelm Fenner, and which included Ernst Witt, Georg Aumann, Oswald Teichmueller and , to form the backbone of the new mathematical research department in the late 1930s, which would eventually be called Section IVc of Cipher Department of the High Command of the Wehrmacht. (abbr. OKW/Chi). The group was led by the German professor of mathematics Wolfgang Franz.