Frederick Charles Louis Constantine, Prince and Landgrave of Hesse (1 May 1868 – 28 May 1940), Friedrich Karl Ludwig Konstantin Prinz und Landgraf von Hessen-Kassel in German, was the brother-in-law of the German Emperor Wilhelm II.
He was elected King of Finland on 9 October 1918, but renounced the throne on 14 December 1918.
During his brief reign he was known in Finnish as Frederik Kaarle, King of Finland.