Nonconformists are Protestant Christians who do not "conform" to the governance and usages of the established church in England, and in Wales until 1914, the Church of England.

Use of the term Nonconformist in England and Wales was precipitated by the Restoration of the Stuart monarchy in 1660, when the Act of Uniformity 1662 renewed opposition to reforms within the established church. By the late 19th century the term specifically included other Reformed Christians (English Presbyterians and Congregationalists), plus the Baptists, Brethren, Methodists, and Quakers. English Dissenters, such as the Puritans, who violated the Act of Uniformity 1558 – typically by practising radical, sometimes separatist, dissent – were retrospectively labelled as Nonconformists. In Ireland, the comparable term until the Church of Ireland's disestablishment in 1869 was Dissenter (the term earlier used in England), commonly referring to Irish Presbyterians who dissented from the approved Anglican communion.

By law and social custom, Nonconformists were restricted from many spheres of public life – not least, from access to public office, civil service careers, or degrees at university – and were referred to as suffering from civil disabilities. In England and Wales in the late 19th century the new terms "free church" and "Free churchman" (or "Free church person") started to replace Nonconformist or Dissenter.

One influential Nonconformist minister was Matthew Henry, who beginning in 1710 published his multi-volume biblical commentary that is still used and available in the 21st century. Isaac Watts is an equally recognised Nonconformist minister whose hymns are still sung by Christians worldwide.

The term is used in a broader sense to refer to Christians who are not communicants of a majority national church, such as the Evangelical Lutheran Church of Sweden.

The Act of Uniformity 1662 required churchmen to use all rites and ceremonies as prescribed in the Book of Common Prayer. It also required episcopal ordination of all ministers of the Church of England – a pronouncement most odious to the Puritans, the faction of the church which had come to dominance during the English Civil War and the Interregnum. Consequently, nearly 2,000 clergy were "ejected" from the established church for refusing to comply with the provisions of the act, an event referred to as the Great Ejection. The Great Ejection created an abiding public consciousness of nonconformity.

Thereafter, a Nonconformist was any English subject belonging to a non-Anglican church or a non-Christian religion. More broadly, any person who advocated religious liberty was typically called out as Nonconformist. The strict religious tests embodied in the laws of the Clarendon Code and other penal laws excluded a substantial section of English society from public affairs and benefits, including certification of university degrees, for well more than a century and a half. Culturally, in England and Wales, discrimination against Nonconformists endured even longer.

Presbyterians, Congregationalists, Baptists, Calvinists, other "reformed" groups and less organised sects were identified as Nonconformists at the time of the 1662 Act of Uniformity. Following the act, other groups, including Methodists, Unitarians, Quakers, Plymouth Brethren, and the English Moravians were officially labelled as Nonconformists as they became organised.