Philadelphia, commonly referred to as Philly, is the most populous city in the U.S. state of Pennsylvania.^[11] Philadelphia is the nation's sixth-most populous city, with a population of 1,603,797 in the 2020 census and is the urban core of the larger Delaware Valley (or Philadelphia metropolitan area), the nation's seventh-largest and one of the world's largest metropolitan regions consisting of 6.245 million residents in the metropolitan statistical area and 7.366 million residents in its combined statistical area.^[12] Philadelphia is known for its extensive contributions to United States history, especially the American Revolution, and served as the nation's capital until 1800.^[13] It maintains contemporary influence in business and industry, culture, sports, and music.^[14][15]

Philadelphia was founded in 1682 by William Penn, an English Quaker and advocate of religious freedom. The city served as the capital of the Pennsylvania Colony during the British colonial era^[3][16] and went on to play a historic and vital role as the central meeting place for the nation's founding fathers. Philadelphia hosted the First Continental Congress in 1774, preserved the Liberty Bell, and hosted the Second Continental Congress during which the founders signed the Declaration of Independence.^[17] The U.S. Constitution was later ratified in Philadelphia at the Philadelphia Convention of 1787. Philadelphia remained the nation's largest city until 1790, served as the nation's first capital from May 10, 1775, until December 12, 1776, and on four subsequent occasions during and following the American Revolution, including from 1790 to 1800 during the construction of the new national capital of Washington, D.C.

With 18 four-year universities and colleges, Philadelphia is one of the nation's leading centers for higher education and academic research.^[18][19] As of 2018, the Philadelphia metropolitan area was the state's largest and nation's ninth-largest metropolitan economy with a gross metropolitan product of US$444.1 billion. The city is home to five Fortune 500 corporate headquarters as of 2022.^[20] As of 2023, metropolitan Philadelphia ranks among the top five U.S. venture capital centers, facilitated by its proximity to New York City's entrepreneurial and financial ecosystems.^[21] The Philadelphia Stock Exchange, owned by Nasdaq since 2008, is the nation's oldest stock exchange and a global leader in options trading.^[22] 30th Street Station, the city's primary rail station, is the third-busiest Amtrak hub in the nation, and the city's multimodal transport and logistics infrastructure also includes Philadelphia International Airport and the rapidly-growing PhilaPort seaport.^[23]

Philadelphia is a national cultural center, hosting more outdoor sculptures and murals than any other city in the nation.^[24][25] Fairmount Park, when combined with adjacent Wissahickon Valley Park in the same watershed, is 2,052 acres (830 ha), representing one of the nation's largest and the world's 45th-largest urban park.^[26] The city is known for its arts, culture, cuisine, and colonial and Revolution-era history; in 2016, it attracted 42 million domestic tourists who spent $6.8 billion, representing $11 billion in economic impact to the city and its surrounding Pennsylvania counties.^[27] With five professional sports teams and one of the nation's most loyal fan bases, Philadelphia is often ranked as the nation's best city for professional sports fans.^{[28][29][30][31]} The city has a culturally and philanthropically active LGBTQ+ community. Philadelphia also has played an immensely influential historic and ongoing role in the development and evolution of American music, especially R&B, soul, and rock.^[32][33]

Philadelphia is a city of many firsts, including the nation's first library (1731),^[34] hospital (1751),^[34] medical school (1765),^[35] national capital (1774),^[36] university (by some accounts) (1779),^[37] central bank (1781), stock exchange (1790),^[34] zoo (1874),^[38] and business school (1881).^[39] Philadelphia contains 67 National Historic Landmarks, including Independence Hall.^[40][41][19] From the city's 17th century founding through the present, Philadelphia has been the birthplace or home to an extensive number of prominent and influential Americans. In 2021, Time magazine named Philadelphia one of the world's greatest 100 places.^[42] Two years later, in 2023, travel guide publisher Lonely Planet ranked Philadelphia the best city in the nation to visit.