Pennsylvania , officially the Commonwealth of Pennsylvania, is a state spanning the Mid-Atlantic, Northeastern, and Appalachian regions of the United States. It borders Delaware to the southeast, Maryland to the south, West Virginia to the southwest, Ohio to the west, Lake Erie and the Canadian province of Ontario to the northwest, New York to the north, and New Jersey to the east.

Of the 50 U.S. states, Pennsylvania is the 33rd-largest by area and the fifth-most populous, with over 13 million residents as of 2020; it subsequently ranks ninth in population density. Nearly half the population is concentrated in the southeastern Delaware Valley, centered around the state's largest city, Philadelphia ; another one-third live in Greater Pittsburgh in the southwest. The state capital and 15th-largest city is Harrisburg; other major cities include Allentown, Scranton, Lancaster, and Erie.

Pennsylvania's geography is highly diverse: the Appalachian Mountains run through its center, while the Allegheny and Pocono Mountains span much of the northeast; close to 60% of the state is forested. While it has only 140 miles of waterfront along Lake Erie and the Delaware River, Pennsylvania has more navigable rivers than any other state, including the Delaware, Ohio, Potomac, and Pine Creek.

Pennsylvania's was one of the thirteen British colonies that would eventually form the United States. It was founded in 1681 through royal land grant to William Penn, son of the state's namesake; the southeast portion was once part of the colony of New Sweden. Established as a haven for religious and political tolerance, the Province of Pennsylvania was noteworthy for its relatively peaceful relations with native tribes, innovative government system, and religious pluralism. Pennsylvania's governing framework inspired the U.S. Constitution, which, along with the Declaration of Independence, was drafted in Independence Hall in Philadelphia; the city also hosted the first and second Constitutional Convention that led the American Revolution. Pennsylvania became the second state to ratify the Constitution on December 12, 1787.