Gellionnen Chapel is a Unitarian place of worship near Pontardawe, South Wales, United Kingdom. The chapel was first built in 1692 by Protestant dissenters, becoming Unitarian in the late 18th century. It is a member of the General Assembly of Unitarian and Free Christian Churches, the umbrella body for British Unitarians. Gellionnen Chapel is the oldest Dissenting chapel in the Swansea Valley, is one of the oldest surviving chapels in the region and is a Grade II* listed building.

Near to the location of the present-day chapel stood a Celtic Church, Llan Eithrim, close to an ancient well of religious significance, Fynnon Wen. Llan Eithrim was consecrated by the Saxon Bishop Herewald in 1060 AD. A Welsh priest named Gwidyr was in charge of the church. Llan Eithrim is believed to have been destroyed during the Reformation.

A stone tablet known as the Gellionnen Stone survived from the old Llan Eithrim church and was incorporated into the wall of Gellionnen Chapel. The Gellionnen Stone, dated c. 900 AD, was donated by the congregation to Swansea Museum in 1965.

In the 1650s, dissenting Protestant preachers visited the area, speaking at local farms. Among them apparently was Jenkin Jones, Catabaptist and Presbyterian Minister and veteran of Oliver Cromwell's parliamentarian army in the English Civil War. In 1662, Reverend Robert Thomas was ejected from Cadoxton Church and established a congregation at Gellionnen, meeting at first in local homes and farms.

Gellionnen Chapel was first built in 1692 by Protestant dissenters on Mynydd Gellionnen (Gellionnen Mountain), who had apparently previously met in local houses and farms. The land was donated by Bussy Mansell, a landowner, Member of Parliament, and veteran of the English Civil War. Gellionnen was one of five Dissenting chapels established under the leadership of Reverend Robert Thomas after his expulsion from the Church of England. The other places of worship in this group included Maesyrhaf, Mynyddbach (Tyrdwncyn), Cwmllynfell, and Blaengwrach, the latter of which also later became Unitarian.

In the late 1690s and early 1700s, Reverend Lewis Davies, who came from Llanedi in Carmarthenshire, is thought to have established a Sunday School at Gellionnen Chapel, which existed for around forty years. His co-minister from 1701, Llewellyn Bevan, was a moderate Calvinist, considered evangelical in his views and democratic in his views. The minister from 1712 to 1742 was Roger Howell, a blacksmith renowned for his knowledge of the scriptures. Howell led the congregation towards Arminianism, preparing the way for the future of development of the chapel towards Unitarianism. In 1715, Gellionnen and its sister chapel, Gwynfe Chapel (10 miles north), had a congregation of around 550. Howell's successor as minister, Joseph Simmonds, ran a school in Llansamlet, Swansea, where the radical preacher, philosopher and mathematician Richard Price was educated.

In the 1740s and 1750s, Arminianism and Unitarianism began to develop among the Gellionnen congregation.

In 1764, Reverend Josiah Rees became minister at Gellionnen Chapel and oversaw its transition towards Unitarianism and the re-building of the chapel in 1801–02. Josiah was the son of Reverend Owen Rees of Hen dy Cwrdd Trecynon (The Old Meeting House, Trecynon) in Aberdare. Josiah was a progressive thinker and studied under Solomon Harries in Swansea and Samuel Thomas, a liberal tutor at Carmarthen.