Karl von Keissler (13 March 1872, Vienna – 9 January 1965) was an Austrian lichenologist and mycologist.
In 1895 he obtained his doctorate from the University of Vienna, afterwards spending several years as an assistant in the botanical garden at Vienna.
From 1899 to 1938, he performed various functions in the botanical department at the Naturhistorisches Museum.
In 1922 (or 1923) he was appointed director of the museum's botanical department.The lichenicolous fungi genus Keissleriomyces bears his name, as do the mycological genera Keissleria, Keissleriella (both named by Franz Xaver Rudolf von Höhnel), Keisslerina and Neokeissleria (both named by Franz Petrak).