Count Lev Nikolayevich Tolstoy (/ˈtoʊlstɔɪ, ˈtɒl-/; Russian: Лев Николаевич Толстой, IPA: [ˈlʲef nʲɪkɐˈla(j)ɪvʲɪtɕ tɐlˈstoj] ^ⓘ; 9 September [O.S. 28 August] 1828 – 20 November [O.S. 7 November] 1910), usually referred to in English as Leo Tolstoy, was a Russian writer. He is regarded as one of the greatest and most influential authors of all time.

Born to an aristocratic family, Tolstoy achieved acclaim in his twenties with his semi-autobiographical trilogy, Childhood, Boyhood and Youth (1852–1856), and with Sevastopol Sketches (1855), based on his experiences in the Crimean War. His War and Peace (1869), Anna Karenina (1878), and Resurrection (1899), which is based on his "youthful sins," are often cited as pinnacles of realist fiction and three of the greatest novels ever written. His oeuvre includes short stories such as "Alyosha the Pot" (1911) and "After the Ball" (1911) and novellas such as Family Happiness (1859), The Death of Ivan Ilyich (1886), The Kreutzer Sonata (1889), The Devil (1911), and Hadji Murat (1912). He also wrote plays and essays concerning philosophical, moral and religious themes.

In the 1870s, Tolstoy experienced a profound moral crisis, followed by what he regarded as an equally profound spiritual awakening, as outlined in his non-fiction work Confession (1882). His literal interpretation of the ethical teachings of Jesus, centering on the Sermon on the Mount, caused him to become a fervent Christian anarchist and pacifist. His ideas on nonviolent resistance, expressed in such works as The Kingdom of God Is Within You (1894), had a profound impact on such pivotal 20th-century figures as Mahatma Gandhi, Ludwig Wittgenstein, Martin Luther King Jr., and James Bevel. He also became a dedicated advocate of Georgism, the economic philosophy of Henry George, which he incorporated into his writing, particularly in his novel Resurrection (1899).

Tolstoy received praise from countless authors and critics, both during his lifetime and after. Virginia Woolf called Tolstoy "the greatest of all novelists", and Gary Saul Morson referred to War and Peace as the greatest of all novels. He received nominations for the Nobel Prize in Literature every year from 1902 to 1906 and for the Nobel Peace Prize in 1901, 1902, and 1909. Tolstoy never being awarded a Nobel Prize remains a major Nobel Prize controversy.

The Tolstoys were a well-known family of old Russian nobility who traced their ancestry to a mythical nobleman named Indris described by Pyotr Tolstoy as arriving "from Nemec, from the lands of Caesar" to Chernigov in 1353 along with his two sons Litvinos (or Litvonis) and Zimonten (or Zigmont) and a druzhina of 3000 people. Indris was then converted to Eastern Orthodoxy, under the name of Leonty, and his sons as Konstantin and Feodor. Konstantin's grandson Andrei Kharitonovich was nicknamed Tolstiy (translated as fat) by Vasily II of Moscow after he moved from Chernigov to Moscow.

Because of the pagan names and the fact that Chernigov at the time was ruled by Demetrius I Starshy, some researchers concluded that they were Lithuanians who arrived from the Grand Duchy of Lithuania. At the same time, no mention of Indris was ever found in the 14th-to-16th-century documents, while the Chernigov Chronicles used by Pyotr Tolstoy as a reference were lost. The first documented members of the Tolstoy family also lived during the 17th century, thus Pyotr Tolstoy himself is generally considered the founder of the noble house, being granted the title of count by Peter the Great.

Tolstoy was born at Yasnaya Polyana, a family estate 12 kilometres (7.5 mi) southwest of Tula, and 200 kilometres (120 mi) south of Moscow. He was the fourth of five children of Count Nikolai Ilyich Tolstoy (1794–1837), a veteran of the Patriotic War of 1812, and Princess Mariya Tolstaya (née Volkonskaya; 1790–1830). His mother died when he was two and his father when he was nine. Tolstoy and his siblings were brought up by relatives. In 1844, he began studying law and oriental languages at Kazan University, where teachers described him as "both unable and unwilling to learn". Tolstoy left the university in the middle of his studies, returned to Yasnaya Polyana and then spent much time in Moscow, Tula and Saint Petersburg, leading a lax and leisurely lifestyle. He began writing during this period, including his first novel Childhood, a fictitious account of his own youth, which was published in 1852. In 1851, after running up heavy gambling debts, he went with his older brother to the Caucasus and joined the army. Tolstoy served as a young artillery officer during the Crimean War and was in Sevastopol during the 11-month-long siege of Sevastopol in 1854–55, including the Battle of the Chernaya. During the war he was recognised for his courage and promoted to lieutenant. He was appalled by the number of deaths involved in warfare, and left the army after the end of the Crimean War.

His experience in the army, and two trips around Europe in 1857 and 1860–61 converted Tolstoy from a dissolute and privileged society author to a non-violent and spiritual anarchist. Others who followed the same path were Alexander Herzen, Mikhail Bakunin and Peter Kropotkin. During his 1857 visit, Tolstoy witnessed a public execution in Paris, a traumatic experience that marked the rest of his life. In a letter to his friend Vasily Botkin, Tolstoy wrote: "The truth is that the State is a conspiracy designed not only to exploit, but above all to corrupt its citizens ... Henceforth, I shall never serve any government anywhere." Tolstoy's concept of non-violence or ahimsa was bolstered when he read a German version of the Tirukkural. He later instilled the concept in Mahatma Gandhi through his "A Letter to a Hindu" when young Gandhi corresponded with him seeking his advice.