Nicholas I (Николай I Павлович, r Nikolai I Pavlovich; 6 July [O.S. 25 June] 1796 – 2 March [O.S. 18 February] 1855) was the Emperor of Russia from 1825 until 1855. He was also the King of Poland and Grand Duke of Finland. He is best known as a political conservative whose reign was marked by geographical expansion, repression of dissent, economic stagnation, poor administrative policies, a corrupt bureaucracy, and frequent wars that culminated in Russia's disastrous defeat in the Crimean War of 1853-56. His biographer Nicholas V. Riasanovsky says that Nicholas displayed determination, singleness of purpose, and an iron will, along with a powerful sense of duty and a dedication to very hard work. He saw himself as a soldier – a junior officer totally consumed by spit and polish. A handsome man, he was highly nervous and aggressive. Trained as an engineer, he was a stickler for minute detail. His reign had an ideology called "Official Nationality" that was proclaimed officially in 1833. It was a reactionary policy based on orthodoxy in religion, autocracy in government, and Russian nationalism.

He was the younger brother of his predecessor, Alexander I. Nicholas inherited his brother's throne despite the failed Decembrist revolt against him and went on to become the most reactionary of all Russian leaders. His aggressive foreign policy involved many expensive wars, having a disastrous effect on the empire's finances.

He was successful against Russia's neighbouring southern rivals as he seized the last territories in the Caucasus held by Persia (comprising modern day Armenia and Azerbaijan) by successfully ending the Russo-Persian War (1826–28). By now, Russia had gained what is now Dagestan, Georgia, Azerbaijan and Armenia from Persia, and had therefore at last gained the clear upper hand in the Caucasus, both geo-politically as well as territorially. He ended the Russo-Turkish War (1828–29) successfully as well. Later on, however, he led Russia into the Crimean War (1853–56) with disastrous results. Historians emphasize that his micromanagement of the armies hindered his generals, as did his misguided strategy. Fuller notes that historians have frequently concluded that "the reign of Nicholas I was a catastrophic failure in both domestic and foreign policy." On the eve of his death, the Russian Empire reached its geographical zenith, spanning over 20 million square kilometers (7.7 million square miles), but in desperate need of reform.