Sergey Gennadiyevich Nechayev (Нечаев, Сергей Геннадиевич in Russian) (October 2,1847 — December 3,1882), was a Russian revolutionary figure.

Born in the then village of Ivanovo, Nechayev moved to Moscow in 1865. A year later, he moved to St.Petersburg, passed a teacher's exam and began teaching at a parish school. Nechayev attended lectures at St Petersburg University (without being officially enrolled) and participated in student unrests in 1868-1869, leading a radical minority together with Petr Tkachev and others. Nechayev took part in devising this student movement's "Program of revolutionary activities", which stated a social revolution as its final goal some time in the spring of 1870. The program also suggested ways for creating a clandestine revolutionaty organization and conducting subversive activities. In particular, the program envisioned composition of the "Catechism of a revolutionary", which would be written by Nechayev in the summer of 1869. The main principle of the "Catechism" - "the end justifies the means" - became Nechayev's slogan from the very start of his revolutionary career.

In January of 1869, Nechayev spread false rumors of his arrest in St.Petersburg, then left for Moscow and went abroad. In Geneva, he pretended to be a representative of some revolutionaty committee, who had fled from the Peter and Paul Fortress, and won the confidence of Mikhail Bakunin and Nikolai Ogarev . Together, they organized a propaganda campaign, financed by Ogarev (despite Alexander Hertsen's strong opposition) from the fund, which had been intended for subsidizing their own revolutionaty activities (the so called "Bakhmetev Fund"). Upon his return to Moscow in 1869, Nechayev pretended to be a proxy of the Russian department of the "Worldwide Revolutionary Union" (which didn't exist) and created an affiliate of some secret society called People's Reprisal (Народная расправа), which had supposedly been existing for quite some time in every corner of Russia. Facing distrust and opposition on the part of one of the members of his organization I.I.Ivanov, Nechayev accused him of betrayal and killed him with the help from 4 of his friends on November 21, 1869.

In late November, Nechayev left for Petersburg, where he tried to continue his activities to create a clandestine society. On December 15, 1869, he had to flee to Switzerland due to the arrests (see Trial of Nechayevtsi ). After having received the second half of the "Bakhmetev Fund", Nechayev published a number of proclamations, aimed at different strata of the Russian population. Together with Ogarev, he published the Kolokol magazine (April-May, 1870, issues 1 to 6). In his article "The Fundamentals of the Future Social System" (Главные основы будущего общественного строя), published in the People's Reprisal (1870, №2), Nechayev shared his vision of the communist system, which Karl Marx and Friedrich Engels would later call a "barracks communism".

Nechayev's abuse of the name of the First International forced its General Council to officially dissociate themselves from him. Nechayev's theoretical unscrupulousness and his bewildering and instigating methods, later exposed by an International member German Lopatin , forced Ogarev and Bakunin to sever their relations with him in the summer of 1870. In September of 1870, Nechayev published an issue of the Commune magazine in London and later, hiding from the tsarist police, went underground in Paris and then Zurich. He also kept in touch with the Polish blanquists, such as Caspar Turski and others. On August 14, 1872, Nechayev was arrested in Zurich and handed over to the Russian police as a criminal. On January 8, 1873, he was sentenced to 20 years of katorga for killing Ivanov. Nechayev was locked up in a ravelin of the Peter and Paul Fortress, where he would bend the guards to his will in the late 1870s. In December of 1880, Nechayev established contact out of his cell with the Executive Committee of Narodnaya volya and proposed a plan for his escape. However, he abandoned the plan due to his unwillingness to distract the efforts of the members of Narodnaya volya from attempting to assassinate Alexander II. In 1882, Nechayev died in his cell.

Despite his personal courage and fanatical dedication to the revolutionary cause, Nechayev is generally considered to have resorted to inadmissible methods, causing harm to the Russian revolutionary movement by endangering clandestine organizations. His methods would later be called Nechayevschina .