Konstantin Pavlovich (Russian: Константи́н Па́влович; 8 May [O.S. 27 April] 1779 – 27 June [O.S. 15 June] 1831) was a grand duke of Russia and the second son of Emperor Paul I and Sophie Dorothea of Württemberg. He was the heir-presumptive for most of his elder brother Alexander I's reign, but had secretly renounced his claim to the throne in 1823. For 25 days after the death of Alexander I, from 19 November (O.S.)/1 December 1825 to 14 December (O.S.)/26 December 1825 he was known as His Imperial Majesty Konstantin I Emperor and Sovereign of Russia, although he never reigned and never acceded to the throne. His younger brother Nicholas became Tsar in 1825. The succession controversy became the pretext of the Decembrist revolt.

Relationships Catherine arranged Konstantin's marriage as she had Alexander's; Juliane of Saxe-Coburg-Saalfeld, 14, and Konstantin, 16, were married on 26 February 1796. As Caroline Bauer recorded in her memoirs, “The brutal Constantine treated his consort like a slave. So far did he forget all good manners and decency that, in the presence of his rough officers, he made demands on her, as his property, which will hardly bear being hinted of.” Due to his violent treatment and suffering health problems as a result, Juliane separated from Konstantin in 1799; she eventually settled in Switzerland. An attempt by Konstantin in 1814 to convince her to return broke down in the face of her firm opposition.

After 19 years of separation, the marriage of Konstantin and Juliane was formally annulled on 20 March 1820. Two months later, on 27 May, Konstantin married the Polish Countess Joanna Grudzińska, who was given the title of Her Serenity Princess of Lowicz. Connected with this, he renounced any claim to the Russian succession, which was formally completed in 1822. After this marriage, he became increasingly attached to his new home of Poland