Allegheny City was a municipality that existed in the U.S. state of Pennsylvania from 1788 until it was annexed by Pittsburgh in 1907. It was located north across the Allegheny River from downtown Pittsburgh, with its southwest border formed by the Ohio River, and is known as the North Side. The city's waterfront district, along the Allegheny and Ohio rivers, became Pittsburgh's North Shore neighborhood.

The boundary of Allegheny City encompassed the modern Pittsburgh neighborhoods of Allegheny Center, Allegheny West, Brighton Heights, California-Kirkbride, Central Northside, Chateau, East Allegheny, Fineview, Manchester, Marshall-Shadeland, North Shore, Perry South, Spring Hill–City View, and Troy Hill; most of Perry North; and parts of Northview Heights and Spring Garden.

Allegheny was laid out in 1788 according to a plan by John Redick. The lots were sold in Philadelphia by Pennsylvania's state government or given as payment to Revolutionary War veterans. It was incorporated as a borough in 1828 and as a city in 1840. Prior to the 1850s, most of the area was still largely farmland, but was subdivided into residential lots, first for the growing German population and later for Croat immigrants. It was commonly referred to as "Deutschtown", derived from the German word Deutsch, referring to the language and ethnicity.

Allegheny annexed adjoining municipalities such as the boroughs of Manchester, Spring Garden and Duquesne, and the areas that became Brighton Heights, Observatory Hill, Perry Hilltop, Summer Hill, Spring Hill and Troy Hill.

The annexation of Allegheny by Pittsburgh began with voting in favor of a "Greater Pittsburg" in a special election held on June 12, 1906 and was effected on December 9, 1907, in the wake of the U.S. Supreme Court's landmark decision Hunter v. City of Pittsburgh handed down on November 19 that year. It was approved by the federal government in 1911. The annexation was controversial at the time, as an overwhelming majority of Allegheny City residents were opposed to the merger. Previous Pennsylvania law had directed that a majority of the voters in each merging municipality had to approve an annexation agreement. In 1906, the State Assembly passed a new law that authorized annexations if a majority of the total voters in both combined municipalities approved the merger. The annexation was rejected by the residents of Allegheny City by a 2:1 margin, but was approved by much more populous Pittsburgh residents, and the annexation bill passed into law. Allegheny City residents tried unsuccessfully for years to have the annexation overturned in court.

The population of Pittsburgh rose from 321,616 in 1900 to 533,905 in 1910, which included the 132,283 who lived in Allegheny in 1910, when the last separate census of Allegheny was taken.

When the two cities were joined, both of the old ward systems were discarded. A new ward system was established made up of 27 wards. In the new ward system, Allegheny was divided into wards 21 to 27. Its past territory is easily seen by viewing a map of the city wards.

In the 1960s, Pittsburgh undertook a massive urban redevelopment project that demolished the historic core of Allegheny City, leaving only the Commons of Allegheny Center and its surrounding neighborhoods. The Carnegie Library, the Old Post Office Building, and the Buhl Planetarium buildings were not demolished. Major portions of the neighborhoods of Allegheny West, Manchester, Central Northside, California-Kirkbride (Old Allegheny Rows), and East Allegheny are listed on the National Register of Historic Places, including the Mexican War Streets in Central Northside.