United and uniting churches are churches formed from the merger or other form of union of two or more different Protestant denominations.

Perhaps the oldest example of a united church is found in Germany, where the Evangelical Church in Germany is a federation of Lutheran, United and Reformed churches, a union dating back to 1817 in some parts of Germany (see Prussian Union). The first union was at a synod in Idstein to form the Protestant Church in Nassau in August 1817, commemorated in naming the church of Idstein Unionskirche 100 years later.

Around the world, each united or uniting church comprises a different mix of predecessor denominations. Trends are visible, however, as most united and uniting churches have one or more predecessors with heritage in the Reformed tradition (either Presbyterian, Congregationalist, or both) and many are members of the World Alliance of Reformed Churches.

In the 1950s and 1960s an ecumenical spirit emerged in many churches in the United States of America, leading to a conciliar movement known in some circles as Conciliarity. A product of this movement was the Consultation on Church Union (COCU). COCU disbanded formally in 2002 but moved into the Churches Uniting in Christ movement.