Back Bay is an officially recognized historic neighborhood of Boston, Massachusetts, built on reclaimed land in the Charles River basin. Construction began in 1859, as the demand for luxury housing exceeded the availability in the city at the time, and the area was fully built by around 1900. It is most famous for its rows of Victorian brownstone homes—considered one of the best preserved examples of 19th-century urban design in the United States—as well as numerous architecturally significant individual buildings, and cultural institutions such as the Boston Public Library, and Boston Architectural College. Initially conceived as a residential-only area, commercial buildings were permitted from around 1890, and Back Bay now features many office buildings, including the John Hancock Tower, Boston's tallest skyscraper. It is also considered a fashionable shopping destination (especially Newbury and Boylston Streets, and the adjacent Prudential Center and Copley Place malls) and home to several major hotels.

The Neighborhood Association of the Back Bay considers the neighborhood's bounds to be "Charles River on the North; Arlington Street to Park Square on the East; Columbus Avenue to the New York New Haven and Hartford right-of-way (South of Stuart Street and Copley Place), Huntington Avenue, Dalton Street, and the Massachusetts Turnpike on the South; Charlesgate East on the West."

Before its transformation into buildable land by a 19th-century filling project, the Back Bay was a bay, west of the Shawmut Peninsula (on the far side from Boston Harbor) between Boston and Cambridge, the Charles River entering from the west. This bay was tidal: the water rose and fell several feet over the course of each day, and at low tide much of the bay's bed was exposed as a marshy flat. As early as 5,200 years before present, Native Americans built fish weirs here, evidence of which was discovered during subway construction in 1913 (see Ancient Fishweir Project and Boylston Street Fishweir).

In 1814, the Boston and Roxbury Mill Corporation was chartered to construct a milldam, which would also serve as a toll road connecting Boston to Watertown, bypassing Boston Neck. The dam prevented the natural tides from flushing sewage out to sea, creating severe sanitary and odor problems. With costs higher and power lower than expected, in the end, the project was an economic failure, and in 1857 a massive project was begun to "make land" by filling the area enclosed by the dam.

The firm of Goss and Munson extended railroad lines to quarries in Needham, Massachusetts, 9 miles (14 km) away; a 35-car train carrying gravel and other fill arrived every 45 minutes, day and night. When the Needham gravel pits were exhausted, additional fill was found in Canton, Dedham, Hyde Park, and Westwood. William Dean Howells recalled "the beginnings of Commonwealth Avenue, and the other streets of the Back Bay, laid out with their basements left hollowed in the made land, which the gravel trains were yet making out of the westward hills."

Present-day Back Bay itself was filled by 1882; the project reached existing land at what is now Kenmore Square in 1890, and was completed in 1900 in the area now known as the Back Bay Fens. Much of the old mill dam remains buried under present-day Beacon Street. The project was the largest of a number of land reclamation projects which, beginning in 1820, more than doubled the size of the original Shawmut Peninsula.

Completion of the Charles River Dam in 1910 converted the former Charles estuary into a freshwater basin; the Charles River Esplanade was constructed to allow residents to enjoy the view of the new lagoon. The Esplanade has since undergone several changes, including the construction of Storrow Drive.

The Back Bay is traversed by five east–west corridors: Beacon Street, Marlborough Street, Commonwealth Avenue, Newbury Street and Boylston Street. These are interrupted at regular intervals by north–south streets named alphabetically: Arlington (along the western border of the Boston Public Garden), Berkeley, Clarendon, Dartmouth, Exeter, Fairfield, Gloucester and Hereford Streets. All of the west–east streets, except Commonwealth Avenue, are one-way streets.