Nicholas I (Russian: Николай I Павлович), (6 July [O.S. 25 June] 1796 – 2 March [O.S. 18 February] 1855), was the Emperor of Russia from 1825 until 1855, known as one of the most reactionary of the Russian monarchs. On the eve of his death, the Russian Empire reached its historical zenith spanning over 20 million square kilometers (7.7 million square miles). In his capacity as the emperor he was also the King of Poland and the Grand Prince of Finland.

Nicholas I was born in Gatchina to Emperor Paul I and Empress Maria Feodorovna. He was a younger brother to Alexander I of Russia and Grand Duke Constantine Pavlovich of Russia. Nicholas was not brought up to be the Emperor of Russia; he had two elder brothers before him. As such, in 1825, when Alexander I suddenly died of typhus, Nicholas was caught between swearing allegiance to his second eldest brother Constantine Pavlovich and accepting the throne for himself. The interregnum lasted until Constantine Pavlovich, who was in Warsaw at that time, confirmed his refusal. Additionally, on 25 December (13 Old Style) Nicholas issued the manifesto claiming his accession to the throne. That manifesto retroactively named 1 December (19 November Old Style), the date of Alexander I's death, as the beginning of his reign. During this confusion a plot was hatched by the military to overthrow Nicholas and to usurp power. This led to the Decembrist Revolt on 26 December (14 Old Style) 1825, an uprising Nicholas was successful in suppressing.

Nicholas completely lacked his brothers' spiritual and intellectual breadth; he saw his role simply as one paternal autocrat ruling his people by whatever means were necessary. Nicholas I was crowned on 14 December 1825, which fell on a Monday; Russian superstition held that Mondays were unlucky days. This particular Monday dawned as a very cold day with temperatures of eight degrees below freezing on the centigrade thermometer. This was regarded by the Russian people as a bad omen for the coming reign. Coincident with the accession of Nicholas I was a demonstration of 3,000 young Imperial Army officers and other liberal minded citizens. This demonstration was an attempt to force the government to accept a constitution and a representative form of government. Nicolas ordered the army out to smash the demonstration. This "revolt" was quickly put down and became known as the Decembrist Revolt. Having experienced the trauma of the Decembrist Revolt on the very first day of his reign, Nicholas I was determined to restrain Russian society. The Third Section of the Imperial Chancellery ran a huge network of spies and informers with the help of Gendarmes. The government exercised censorship and other controls over education, publishing, and all manifestations of public life.

In 1833 the minister of education, Sergey Uvarov, devised a program of "Autocracy, Orthodoxy, and Nationality" as the guiding principle of the regime. The people were to show loyalty to the unlimited authority of the tsar, to the traditions of the Russian Orthodox Church, and, in a vague way, to the Russian nation. These romantic and conservative principles outlined by Uvarov were also espoused by Vasilii Zhukovskii, one of the tutors of the Grand Duke Alexander. The results of these conservative principles led, broadly speaking, to repression in general and to suppression of non-Russian nationalities and religions in particular. For example, the government suppressed the Greek-Catholic Churches in Ukraine and Belarus in 1839.

Nicholas disliked serfdom and toyed with the idea of abolishing it in Russia, but did not do so for practical reasons of state. He feared the landowners and believed they might turn against him if he abolished serfdom. However, he did make some efforts to improve the lot of the state peasants (serfs owned by the government) with the help of the minister Pavel Kiselev. During most of his reign he tried to increase his control over the landowners and other influential groups in Russia.

The official emphasis on Russian nationalism contributed to a debate on Russia's place in the world, the meaning of Russian history, and the future of Russia. One group, the Westernizers, believed that Russia remained backward and primitive and could progress only through more Europeanization. Another group, the Slavophiles, enthusiastically favored the Slavs and their culture and customs, and had a distaste for westerners and their culture and customs. The Slavophiles viewed Slavic philosophy as a source of wholeness in Russia and were skeptical of Western rationalism and materialism. Some of them believed that the Russian peasant commune, or Mir, offered an attractive alternative to Western capitalism and could make Russia a potential social and moral saviour representing thus a form of Russian messianism.

Despite the repressions of this period, Russia experienced a flowering of literature and the arts. Through the works of Aleksandr Pushkin, Nikolai Gogol, Ivan Turgenev, and numerous others, Russian literature gained international stature and recognition. Ballet took root in Russia after its importation from France, and classical music became firmly established with the compositions of Mikhail Glinka (1804 – 1857).

In foreign policy, Nicholas I acted as the protector of ruling legitimism and as guardian against revolution. His offers to suppress revolution on the European continent, trying to follow the trends of his eldest brother, Tsar Alexander I, earned him the label of gendarme of Europe. In 1825 Nicholas I was crowned and began to limit the liberties of constitutional monarchy in Congress Poland. In return, after the November Uprising broke out, in 1831 the Polish parliament deposed Nicholas as king of Poland in response to his repeated curtailment of its constitutional rights. The Tsar reacted by sending Russian troops into Poland. Nicholas crushed the rebellion, abrogated the Polish constitution, and reduced Poland to the status of a province, Privislinsky Krai, and embarked on a policy of repression towards Catholics.

In 1848, when a series of revolutions convulsed Europe, Nicholas was in the forefront of reaction. In 1849 he intervened on behalf of the Habsburgs to suppress the uprising in Hungary, and he also urged Prussia not to accept a liberal constitution.

While Nicholas was attempting to maintain the status quo in Europe, he adopted an aggressive policy toward the Ottoman Empire. Nicholas I was following the traditional Russian policy of resolving the so-called Eastern Question by seeking to partition the Ottoman Empire and establish a protectorate over the Orthodox population of the Balkans, still largely under Ottoman control in the 1820s. Russia fought a successful war against the Ottomans in 1828 and 1829. In 1833 Russia negotiated the Treaty of Unkiar-Skelessi with the Ottoman Empire. The major European parties mistakenly believed that the treaty contained a secret clause granting Russia the right to send warships through the Bosporus and Dardanelles straits. By the London Straits Convention of 1841, they affirmed Ottoman control over the straits and forbade any power, including Russia, to send warships through the straits. Based on his role in suppressing the revolutions of 1848 and his mistaken belief that he had British diplomatic support, Nicholas moved against the Ottomans, who declared war on Russia October 8, 1853. On November 30, 1853, Russian Admiral Nakhimov caught the Turkish fleet in the harbor at Sinope and destroyed the Turkish fleet.

In 1854, fearing the results of an Ottoman defeat by Russia, Britain, France, the Kingdom of Sardinia, and the Ottoman Empire joined forces in the conflict known as the Crimean War to the Ottomans and Western Europeans, but known in Russia as the Eastern War, Russian: Восточная война, Vostochnaya Vojna (March 1854 – February 1856). In April of 1854, Austria signed a defensive pact with Prussia. Thus, Russia found herself in a war with the whole of Europe allied against her. Austria offered the Ottomans diplomatic support, and Prussia remained neutral, thus leaving Russia without any allies on the continent. The European allies landed in Crimea and laid siege to the well-fortified Russian base at Sevastopol. The Russians lost battles at Alma in September of 1854. This loss was followed by losses in battles at Balaklava and Inkerman. After a year's siege the base fell, exposing Russia's inability to defend a major fortification on its own soil. After the death of Nicholas I, Alexander II became Tsar. On January 15, 1856 and the new Tsar took Russia out of the war on very unfavorable terms which included the loss of the port of Sevastopol.

Nicholas died on 2 March 1855, during the Crimean War. He caught a chill; refusing to rest and recuperate, he persisted with his usual heavy workload, leading to pneumonia and death.

There have been many damning verdicts on Nicholas' rule and legacy. At the end of his life, one of his most devoted civil servants, A.V. Nikitenko, opined that, "The main failing of the reign of Nicholas Pavlovich was that it was all a mistake." However, from time to time, some efforts are made to revive Nicholas' reputation. He believed, it is said, in his own oath and in respecting other people's rights as well as his own; witness Poland before 1831 and Hungary in 1849. It is also said that he hated serfdom at heart and would have liked to destroy it, as well as detesting the tyranny of the Baltic squires over their "emancipated" peasantry. Shortly before his death he made his son Alexander II promise to abolish serfdom. According to Igor Vinogradov, Nicholas and his Minister of Public Education Uvarov spread education through the Empire at all levels. The Kiev University was founded in 1834 by Nicholas.

As a traveler in Spain, Italy, and Russia, the Frenchman Marquis de Custine said in his widely read book Empire of the Czar: A Journey Through Eternal Russia that, inside, Nicholas was a good person, and behaved as he did only because he believed he had to. "If the Emperor, has no more of mercy in his heart than he reveals in his policies, then I pity Russia; if, on the other hand, his true sentiments are really superior to his acts, then I pity the Emperor."

Nicholas is involved in an urban myth about the railroad from Moscow to Saint Petersburg. When it was to be constructed, the engineers proposed to Nicholas that he draw the path of the future railroad on the map himself. So he is said to have taken a ruler and put one end at Moscow, the other at Saint Petersburg, and then drawn a straight line - but his finger was slightly sticking out, and this left the railroad with a small curve. In fact, this curve was added in 1877, 26 years after the railway's construction, to circumvent a steep gradient that lasted for 15 km, and interfered with the railway's functionality. This curving had to be rectified in the early 2000s when the speed of the trains running between the two cities had to be increased.