Islamic extremism, Islamist extremism, or radical Islam, is used in reference to extremist beliefs and behaviors which are associated with the Islamic religion. These are controversial terms with varying definitions, ranging from academic understandings to the idea that all ideologies other than Islam have failed and are inferior to Islam. These terms can also be used in reference to other sects of Islam that do not share such beliefs. Political definitions of Islamic extremism include the one which is used by the government of the United Kingdom, which understands Islamic extremism as any form of Islam that opposes "democracy, the rule of law, individual liberty and mutual respect and tolerance of different faiths and beliefs". In 2019, the U.S. Institute for Peace released an important report on extremism in fragile states that developed recommendations focused on adopting a shared understanding, operationalize a prevention framework, and rallying the international community.

Islamic extremism should not be equated with Islamic fundamentalism or Islamism, the former is defined as a movement of Muslims who believe that Muslim-majority countries should return to the fundamentals of an Islamic state and the latter movement is defined as a form of political Islam, but some experts consider both Islamic fundamentalism and Islamism to be forms of Islamic extremism. Acts of violence which are committed by Islamic terrorists and jihadists are frequently blamed on Islamic extremism, but Islamic extremism is not the root cause of every act of violence.