Louis IV (Friedrich Wilhelm Ludwig Karl) (12 September 1837 – 13 March 1892), was the Grand Duke of Hesse and by Rhine, reigning from 13 June 1877 until his death. Through his own and his children's marriages he was connected to the British Royal Family, to the Imperial House of Russia and to other reigning dynasties of Europe.

Louis was born at the Prinz-Karl-Palais in Darmstadt, the capital of the Grand Duchy of Hesse and by Rhine in the German Confederation, the first son and child of Prince Charles of Hesse and by Rhine (23 April 1809 – 20 March 1877) and Princess Elisabeth of Prussia (18 June 1815 – 21 March 1885), granddaughter of King Frederick William II of Prussia. As his father's elder brother Louis III (1806-1877), the reigning Grand Duke of Hesse and by Rhine, had been married to his first wife since 1833 without legitimate children and from 1868 was married morganatically, Prince Louis was the likely heir eventual to the grand ducal throne from childhood.

On 1 July 1862, Louis married Princess Alice, the third child of Queen Victoria of the United Kingdom, at Osborne House on the Isle of Wight. On the day of the wedding, the Queen issued a royal warrant granting her new son-in-law the style of Royal Highness in the United Kingdom. The Queen also subsequently made Prince Louis a knight of the Order of the Garter.

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