Major Friedrich Wilhelm Kasiski (29 November 1805 – 22 May 1881) was a German infantry officer, cryptographer and archeologist.
Kasiski enlisted in East Prussia's 33rd Infantry Regiment on 20 March 1823 at the age of 17.
The significance of Kasiski's cryptanalytic work was not widely realised at the time, and he turned his mind to archaeology instead.
The later years of his life were spent at Neustettin (Szczecinek); the 11th edition of Encyclopædia Britannica cited a scholarly article by Kasiski in its entry on the town.
Historian David Kahn notes, "Kasiski died on May 22, 1881, almost certainly without realizing that he had written a revolution in cryptology" (The Codebreakers).