Frederick Charles Louis Constantine, Prince and Landgrave of Hesse (German: Friedrich Karl Ludwig Konstantin Prinz und Landgraf von Hessen-Kassel; Finnish: Fredrik Kaarle; 1 May 1868 – 28 May 1940), was the brother-in-law of the German Emperor, Wilhelm II.

Frederick William was of practically Danish upbringing, having lived all his life in Denmark, but in 1875, when the senior branch of Hesse-Kassel became extinct, he settled in northern Germany, where the House had substantial landholdings.

On 25 January 1893, Frederick Charles married Princess Margaret of Prussia, youngest sister of Kaiser Wilhelm II and a granddaughter of Queen Victoria of the United Kingdom.

[1] However, with the end of World War I, in light of his German birth (and despite his Danish roots through his paternal grandmother) and the abdication of Emperor Wilhelm II of Germany ending monarchies in Germany, the arrangement was quickly considered untenable by influential Finns of the time and by Frederick himself.

However, according to certain family documents and correspondence, his successor as King of Finland would have been his second surviving son Prince Wolfgang of Hesse (1896–1989), apparently because Philipp was already the designated heir of the rights over the Electorate of Hesse, but certainly because Wolfgang was with his parents in 1918 and ready to travel to Finland, where a wedding to a Finnish lady was already in preparation for the coming crown prince.