Frederick Charles Louis Constantine, Prince and Landgrave of Hesse (1 May 1868, Gut Panker – 28 May 1940, Kassel), 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.

Frederick was born at his family's manor, Gut Panker, in Plön, Holstein. He was the third son of Frederick William of Hesse, Landgrave of Hesse, and his wife Princess Anna of Prussia, daughter of Prince Charles of Prussia and Princess Marie Louise of Saxe-Weimar-Eisenach. Frederick William, a Danish military officer, had been one (and perhaps the foremost) of the candidates of Christian VIII of Denmark in the 1840s to succeed to the Danish throne if the latter's male line died out, but renounced his rights to the throne in 1851 in favor of his sister, Louise. 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 Great Britain. They had six children, including two sets of twins:

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