Karl Koester (born 2 April 1843 in Bad Dürkheim, died 2 December 1904 in Bonn) was a German pathologist and rector of the University of Bonn from 1898 to 1899.

His doctoral advisor and mentor was Friedrich Daniel von Recklinghausen, and he subsequently worked as Recklinghausen's assistant.

He succeeded Eduard von Rindfleisch as professor of pathology at the University of Bonn in 1874.

In 1868 he published Ueber die feinere Structur der menschlichen Nabelschnur ("On the finer structure of the human umbilical cord").

[3] He became a member of the German National Academy of Sciences Leopoldina in 1880.