Old South Church (also known as New Old South Church or Third Church), is a historic United Church of Christ congregation in Boston, Massachusetts, first organized in 1669. Its present building at 645 Boylston Street in Copley Square, built on newly filled land in the Back Bay section of Boston, was completed in 1873. It was designed in the Gothic Revival style by Charles Amos Cummings and Willard T. Sears, while the campanile was designed by the architects Allen & Collens and added between 1935 and 1937. It was designated a National Historic Landmark in 1970 for its architectural significance as one of the finest High Victorian Gothic churches in New England. It is home to one of the oldest religious communities in the United States.

Established in 1669, Old South Church's congregation is one of the older religious communities in the United States. It was organized by Congregationalist dissenters from Boston's First Church, and it was known as the Third Church, to distinguish it from the First and Second Congregational churches in the city. The Third Church's congregation met first in their Cedar Meeting House (1670), then at the Old South Meeting House (1729) at the corner of Washington and Milk Streets in Boston.

Members of the congregation have included founders Edward Raynsford, Thomas Savage, John Alden, John Hull, Judith Quincy Hull and parishioners Daniel Quincy, John Alden Jr., Samuel Adams, William Dawes, Benjamin Franklin, Samuel Sewall, and Phillis Wheatley. In 1773, Samuel Adams gave the signals from the Old South Meeting House for the "war whoops" that started the Boston Tea Party.

During the Unitarian Movement of the early 19th century, Old South was the sole Congregational Church in Boston to adhere to the doctrine of Trinitarianism. In 1816 Old South Church joined with Park Street Church to form the City Mission Society, a social justice society to serve Boston's urban poor.

During the American Civil War, Old South became a recruiting center for the Union Army under minister Jacob Manning. Though the congregation was not entirely abolitionist, it strongly supported the Union cause. The conclusion of the Civil War was followed by an expansive time of increased inclusion for the congregation. Under minister George Angier Gordon, the congregation moved from its meeting house at Washington Street to its Back Bay location in 1875, occupying the present church constructed on newly filled land.

Old South's commitment to urban ministry and care continued into the 20th century. It has included new members who have been increasingly diverse by race, class, and sexual orientation. The congregation has formally adopted a platform of equality, social justice, and peace.^{[citation needed]}

The church building was designed between 1870 and 1872 by the Boston architectural firm of Cummings and Sears in the Venetian Gothic style. The style follows the precepts of the British cultural theorist and architectural critic John Ruskin (1819–1900) as outlined in his treatise The Stones of Venice. Old South Church in Boston remains one of the most significant examples of Ruskin's influence on American architecture. The architects, Charles Amos Cummings and Willard T. Sears, also designed the Isabella Stewart Gardner Museum. The exterior of the church is primarily built of Roxbury Conglomerate commonly called puddingstone. Many arches, and several walls of stone, are striped with alternating courses of yellow-beige and deep red sandstone. The porticos and large open arches in the campanile are decorated with simple plate tracery. The upper arches of the porticos are decorated with screens of ornate wrought iron. The building is roofed in alternating bands of red and dark gray slate and the roofline finished with ornamental iron cresting.

A tall tower, or campanile, is the trademark feature of Old South and is visible from several Boston neighborhoods. The tower, on the western end of the church, rises to a height of 246' and houses the church's 2,020-pound bell. This is the second campanile built on the same site, designed by Allen & Collens it is similar to the 1875 design in its use of Moorish arches. The first tower, completed in 1875 along with the present Narthex and sanctuary, had begun to list by the late 1920s. The cause was determined to be the faulty footings and piles anchored in the soft former swampland. They were insufficient for the load of the tower. The congregation engaged the architectural firm of Allen & Collens to design a replacement campanile and a new chapel to be named in memory of the Reverend George Angier Gordon. The tower was dismantled, and early 1930s technology of steam shovel and steel pilings provided a lasting solution. Today, the pitch and height of the tower are tested annually and records attest to its enduring stability. The bell wheel, which by motion of a heavy rope swings the large bell, had deteriorated by the late 20th century requiring that the bell be rung by an external hammer. A faithful reconstruction of the original 1931 bell wheel, installed in early fall 2006, returned Old South's bell to "full swing."