Prince Ioann Konstantinovich of Russia (Russian: Иоанн Константинович) (5 July 1886 – 18 July 1918), sometimes also known as Prince John, Prince Ivan or Prince Johan, was the eldest son of Grand Duke Konstantin Konstantinovich of Russia by his wife Yelizaveta Mavrikievna, née Princess Elisabeth of Saxe-Altenburg. He was described by contemporaries as a gentle, religious person, nicknamed "Ioannchik" by his relatives.

Ioann Konstantinovich was born as a Grand Duke of Russia with the style Imperial Highness, but at the age of 9 days, a Ukaz of his cousin Emperor Alexander III of Russia stripped him of that title, as the Ukaz amended the House Law by limiting the grand-ducal title to grandsons of a reigning emperor. As a result, he received the title Prince of the Imperial Blood (Prince of Russia) with the style Highness.

He once entertained the possibility of becoming an Orthodox monk, but eventually fell in love with Princess Helen of Serbia. They married on 2 September 1911, and Helen took the name Princess Yelena Petrovna of Russia. They had a son, Prince Vsevolod Ivanovich (20 January 1914 – 18 June 1973), and a daughter, Princess Yekaterina Ivanovna (12 July 1915 – 13 March 2007), who was the last member of the Imperial Family to be born before the fall of the dynasty, and was ultimately to become the last surviving uncontested dynast of the Imperial House of Russia.

Prince Ioann fought in the First World War, was decorated as a war hero, and was at the front when the Russian Revolution of 1917 started. In April 1918 he was exiled to the Urals by the Bolsheviks, and later murdered in July the same year in a mineshaft near Alapayevsk, along with his brothers Prince Konstantin Konstantinovich and Prince Igor Konstantinovich, his cousin Prince Vladimir Pavlovich Paley, and other relatives and friends.

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