The Conway Hall Ethical Society, formerly the South Place Ethical Society, based in London at Conway Hall, is thought to be the oldest surviving freethought organisation in the world and is the only remaining ethical society in the United Kingdom. It now advocates secular humanism and is a member of Humanists International.

The Society's origins trace back to 1787, as a nonconformist congregation, led by Elhanan Winchester, rebelling against the doctrine of eternal damnation. The congregation, known as the Philadelphians or Universalists, secured their first home at Parliament Court Chapel on the eastern edge of London on 14 February 1793.

William Johnson Fox became minister of the congregation in 1817. By 1821 Fox's congregation had decided to build a new place of worship, and issued a call for "subscriptions for a new Unitarian chapel, South Place, Finsbury".

Subscribers (donors) included businessman and patron of the arts Elhanan Bicknell. In 1824 the congregation built a chapel at South Place, in the Finsbury district of central London. The chapel was repaired by John Wallen, of a family of London architects and builders. This chapel later became the home of South Place Ethical Society. The chapel stood on the site of what is now the office building known as 8 Finsbury Circus; the building has an entrance in South Place which bears a plaque commemorating the chapel.

In 1929 they built new premises, Conway Hall, at 37 (now numbered 25) Red Lion Square, in nearby Bloomsbury, on the site of a tenement, previously a factory belonging to James Perry, a pen and ink maker. Conway Hall is named after an American, Moncure D. Conway, who led the Society from 1864 to 1885 and from 1892 to 1897, during which time it moved further away from Unitarianism. Conway spent the break in his tenure in the United States, writing a biography of Thomas Paine. In 1888 the name of the Society was changed from South Place Religious Society to South Place Ethical Society (SPES) under Stanton Coit's leadership. In 1950 the SPES joined the Ethical Union. In 1969 another name change was mooted, to The South Place Humanist Society, a discussion that sociologist Colin Campbell suggests symbolized the death of the ethical movement in England.

The original name, South Place Ethical Society, was retained until 2012, when it changed to Conway Hall Ethical Society. In November 2013 Elizabeth Lutgendorff was elected Chair of the Conway Hall General Committee, becoming the youngest Chair in the society's history. On 1 August 2014 the society became a Charitable Incorporated Organisation with a new charitable object: "The advancement of study, research and education in humanist ethical principles". This replaced the previous object: "The study and dissemination of ethical principles and the cultivation of a rational religious sentiment."

In 1935 twenty members of the Society signed a document stating that Conway Hall was their regular place of worship. It was therefore certified for marriages by the Registrar-General until 1977 when the Deputy Registrar-General ruled that the Hall could not be used for weddings under the terms of the Places of Worship Registration Act. This followed the report in the winter of 1975 of a marriage solemnised at Conway Hall. He was probably influenced by the 1970 ruling of Lord Denning, that marriages could only be solemnised in places whose principal use is for the "worship of God or [to do] reverence to a deity. Until the ruling the Society had an established tradition of performing secular funerals, memorial ceremonies and namings of children at Conway Hall.

The Sunday Concerts at Conway Hall can be traced back to 1878 when the Peoples Concert Society was formed for the purpose of "increasing the popularity of good music by means of cheap concerts". Many of these concerts were held at the South Place Institute, but in 1887 the Peoples Concert Society had to cut short its season through lack of funds. At that point the South Place Ethical Society undertook the task of organising concerts under the first Honorary Secretary Alfred J. Clements and Assistant Secretary George Hutchinson who continued to run them under the name 'South Place Sunday Concerts'. The thousandth concert was played on 20 February 1927, and the two-thousandth concert was held at the Queen Elizabeth Hall on 9 March 1969.