Sergey Gennadiyevich Nechayev  (October 2, 1847 – November 21 or December 3, 1882) is a Russian communist revolutionary, often associated with the nihilist movement and known for his single-minded pursuit of revolution by any means necessary, including terrorism and revolutionary terror.

He is the author of the radical Catechism of a Revolutionary.

Nechayev flees Russia in 1869 after having been involved in the murder of a former comrade.

Complicated relationships with fellow revolutionaries cause him to be expelled from the International Workingmen's Association.

Arrested in Switzerland in 1872, he is sent back to Russia, where he receives a twenty-year sentence and dies in prison.

The character Pyotr Verkhovensky in Fyodor Dostoevsky's anti-nihilistic novel Demons is based on Nechayev.

Nechayev is often called a "Bolshevik before the Bolsheviks" and many other Russian revolutionaries will be accused of Nechayevshchina by their opponents.

The term is associated with authoritarianism, radicalism and sectarianism in the time that precedes the Russian Revolution of 1917.