Philadelphia (/ˌfɪləˈdɛlfiə/ ^ⓘ FIL-ə-DEL-fee-ə), colloquially referred to as Philly, is the most populous city in the U.S. state of Pennsylvania. It is the sixth-most populous city in the United States with a population of 1.6 million at the 2020 census, while the Philadelphia metropolitan area (sometimes called the Delaware Valley) with 6.33 million residents is the nation's ninth-largest metropolitan area. Philadelphia is known for its culture, cuisine, and history, maintaining contemporary influence in business and industry, culture, sports, and music.

Philadelphia was founded in 1682 by William Penn, an English Quaker and advocate of religious freedom, and served as the capital of the colonial era Province of Pennsylvania. It then played a vital role during the American Revolution and Revolutionary War. It served as the central meeting place for the nation's Founding Fathers in hosting the First Continental Congress (1774) and the Second Continental Congress, during which the Founders formed the Continental Army, elected George Washington as its commander, and adopted the Declaration of Independence on July 4, 1776. During the Revolutionary War's Philadelphia campaign, the city briefly fell to the British Army, which occupied Philadelphia for nine months from September 1777 to June 1778. Following the end of the Revolutionary War, the U.S. Constitution was ratified at the Philadelphia Convention. Philadelphia remained the nation's largest city until 1790, and it served as the nation's first capital from May 10, 1775, until December 12, 1776, and on four subsequent occasions until 1800, when construction of the new national capital in Washington, D.C. was completed.

With 17 four-year universities and colleges in the city, Philadelphia is one of the nation's leading centers for higher education and academic research. The city hosts more outdoor sculptures and murals than any other city in the nation. 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 55th-largest urban park. With five professional sports teams and one of the nation's most loyal and passionate fan bases, Philadelphia is often ranked as the nation's best city for professional sports fans. The city has a culturally and philanthropically active LGBTQ+ community. Philadelphia also has played an influential historic and ongoing role in the development and evolution of American music, especially R&B, soul, and rock.

As of 2023^[update], the Philadelphia metropolitan area had a gross metropolitan product of US$557.6 billion and is home to 13 Fortune 500 corporate headquarters. Metropolitan Philadelphia ranks as one of the nation's Big Five venture capital hubs, facilitated by its proximity to both the financial ecosystems of New York City and the regulatory environment of Washington, D.C. Metropolitan Philadelphia is also a biotechnology hub. The Philadelphia Stock Exchange, owned by Nasdaq since 2008, is the nation's oldest stock exchange and a global leader in options trading. 30th Street Station, the city's primary rail station, is the third-busiest Amtrak hub in the nation with over 4.1 million passengers in 2023. The city's multimodal transportation and logistics infrastructure includes Philadelphia International Airport, the PhilaPort seaport; and Interstate 95, the spine of the north–south highway system along the U.S. East Coast.

Philadelphia is a city of many firsts, including the nation's first library (1731), hospital (1751), medical school (1765), national capital (1774), university (by some accounts) (1779), central bank (1781), stock exchange (1790), zoo (1874), and business school (1881). Philadelphia contains 67 National Historic Landmarks, including Independence Hall. 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.

Prior to the arrival of Europeans in the early 17th century, the Lenape, an Indian tribe also known as the Delaware Indians, lived in the village of Shackamaxon in present-day Philadelphia and the surrounding area. The Lenape historically lived along the Delaware River watershed, western Long Island, and the Lower Hudson Valley. Most Lenape were pushed out of the region during the 18th century as the original Thirteen Colonies expanded, which was further exacerbated by losses from intertribal conflicts. Lenape communities were also weakened by newly introduced diseases, mainly smallpox, and conflicts with Europeans. The Iroquois occasionally fought the Lenape. Surviving Lenape moved west into the upper Ohio River basin. Following the American Revolutionary War and the subsequent establishment of the United States, the Lenape began moving further west. In the 1860s, the U.S. federal government sent most remaining Lenape in the eastern United States to the Indian Territory in present-day Oklahoma and surrounding territories as part of the Indian removal policy.

Europeans first entered Philadelphia and the surrounding Delaware Valley in the early 17th century. The first settlements were founded by Dutch colonists, who built Fort Nassau on the Delaware River in 1623 in what is now Brooklawn, New Jersey. The Dutch considered the entire Delaware River valley to be part of their New Netherland colony. In 1638, Swedish settlers led by renegade Dutch established the colony of New Sweden at Fort Christina, located in present-day Wilmington, Delaware, and quickly spread out in the valley. In 1644, New Sweden supported the Susquehannocks in their war against Maryland colonists. In 1648, the Dutch built Fort Beversreede on the west bank of the Delaware, south of the Schuylkill River near the present-day Eastwick section of Philadelphia, to reassert their dominion over the area. The Swedes responded by building Fort Nya Korsholm, or New Korsholm, named after a town in Finland with a Swedish majority.

In 1655, a Dutch military campaign led by New Netherland Director-General Peter Stuyvesant took control of the Swedish colony, ending its claim to independence. The Swedish and Finnish settlers continued to have their own militia, religion, and court, and to enjoy substantial autonomy under the Dutch. An English fleet captured the New Netherland colony in 1664, though the situation did not change substantially until 1682, when the area was included in William Penn's charter for Pennsylvania.