Left-wing extremism is a form of political extremism associated with the far left of left wing politics. Left-wing extremism almost always has some basis in Marxist or Communist ideology, and because of this pseudo-academic grounding it is often considered to be distinct from Right-wing extremism, which is considered more reactionary and emotive. However, at the same time left-wing extremism is considered to lack any sense of restraint, and it tends to demand solutions that are radical and extreme even by the standards of other Marxists.

Left-wing extremism thus is characterized by a promotion of hate and encouragement of violence or terrorist activities to be carried out against ideological enemies, namely anyone who is seen to serve capitalism or the state. Left-wing extremism is often marked by indiscriminate hostility towards even moderate leftist forces. For example, social democrats are often targeted with as much vigor as conservatives. These organisations may be anarchist in nature, though this classification is not exclusively a leftist phenomenon.

Examples of left-wing extremist groups often include 60's militias groups such as the Weathermen or the Black Panthers as well as Communist revolutionary groups such as the Shining Path or Khmer Rouge.