Christian fundamentalism, also known as fundamental Christianity or fundamentalist Christianity, is a religious movement emphasizing biblical literalism. In its modern form, it began in the late 19th and early 20th centuries among British and American Protestants as a reaction to theological liberalism and cultural modernism. Fundamentalists argued that 19th-century modernist theologians had misunderstood or rejected certain doctrines, especially biblical inerrancy, which they considered the fundamentals of the Christian faith.

Fundamentalists are almost always described as upholding beliefs in biblical infallibility and biblical inerrancy, in keeping with traditional Christian doctrines concerning biblical interpretation, the role of Jesus in the Bible, and the role of the church in society. Fundamentalists usually believe in a core of Christian beliefs, typically called the "Five Fundamentals". These arose from the Presbyterian Church issuance of "The Doctrinal Deliverance of 1910". Topics included are statements on the historical accuracy of the Bible and all of the events which are recorded in it as well as the Second Coming of Jesus Christ.

Fundamentalism manifests itself in various denominations which believe in various theologies, rather than a single denomination or a systematic theology. The ideology became active in the 1910s after the release of The Fundamentals, a twelve-volume set of essays, apologetic and polemic, written by conservative Protestant theologians in an attempt to defend beliefs which they considered Protestant orthodoxy. The movement became more organized within U.S. Protestant churches in the 1920s, especially among Presbyterians, as well as Baptists and Methodists. Many churches which embraced fundamentalism adopted a militant attitude with regard to their core beliefs. Reformed fundamentalists lay heavy emphasis on historic confessions of faith, such as the Westminster Confession of Faith, as well as uphold Princeton theology. Since 1930, many fundamentalist churches in the Baptist tradition (who generally affirm dispensationalism) have been represented by the Independent Fundamental Churches of America (renamed IFCA International in 1996), while many theologically conservative connexions in the Methodist tradition (who adhere to Wesleyan theology) align with the Interchurch Holiness Convention; in various countries, national bodies such as the American Council of Christian Churches exist to encourage dialogue between fundamentalist bodies of different denominational backgrounds. Other fundamentalist denominations have little contact with other bodies.

A few scholars label Catholic activist conservative associations who reject modern Christian theology in favor of more traditional doctrines as fundamentalists. The term is sometimes mistakenly confused with the term evangelical.

The term fundamentalism entered the English language in 1922, and it is often capitalized when it is used in reference to the religious movement. By the end of the 20th century, the term fundamentalism acquired a pejorative connotation, denoting religious fanaticism or extremism, especially when such labeling extended beyond the original movement which coined the term and those who self-identify as fundamentalists.

Some who hold certain, but not all beliefs in common with the original fundamentalist movement reject the label fundamentalism, due to its perceived pejorative nature, while others consider it a banner of pride. In certain parts of the United Kingdom, using the term fundamentalist with the intent to stir up religious hatred is a violation of the Racial and Religious Hatred Act of 2006.

The movement has its origins in 1878 in a meeting of the "Believers' Meeting for Bible Study" (Niagara Bible Conference) in the United States, where 14 fundamental beliefs were established by evangelical pastors.

Fundamentalism draws from multiple traditions in British and American theologies during the 19th century. According to authors Robert D. Woodberry and Christian S. Smith,