Pennsylvania, officially the Commonwealth of Pennsylvania, is a state spanning the Mid-Atlantic, Northeastern, Appalachian, and Great Lakes regions of the United States. It borders Delaware to its southeast, Maryland to its south, West Virginia to its southwest, Ohio and the Ohio River to its west, Lake Erie and New York to its north, the Delaware River and New Jersey to its east, and the Canadian province of Ontario to its northwest via Lake Erie. Pennsylvania's most populous city is Philadelphia.

Pennsylvania was founded in 1681 through a royal land grant to William Penn, the son of the state's namesake. Before that, between 1638 and 1655, a southeast portion of the state was part of New Sweden, a Swedish colony. Established as a haven for religious and political tolerance, the colonial-era Province of Pennsylvania was known for its relatively peaceful relations with native tribes, innovative government system, and religious pluralism.

Pennsylvania later played a vital and historic role in the American Revolution and the ultimately successful quest for independence from the British Empire, hosting the First and Second Continental Congress in Philadelphia, which formed the Continental Army and elected George Washington as its commander in 1775 during the American Revolutionary War, and unanimously adopted the Declaration of Independence the following year. In 1787, following the establishment of the nation's independence, the Constitution of the United States, now the world's oldest and longest-standing written and codified national constitution, was written at the Constitutional Convention in Philadelphia, and was ratified in Philadelphia the following year. On December 12, 1787, Pennsylvania was the second state to ratify the U.S. Constitution.

In July 1863, during the American Civil War, the Battle of Gettysburg, fought over three days in and around Gettysburg, was the bloodiest battle of both the Civil War and of any battle in American military history, with over 50,000 Union and Confederate casualties. But the success of the Union army at Gettysburg in repelling Confederate general Robert E. Lee's attempt to invade the North also proved the Civil War's turning point, leading to the Union's preservation. Recognizing the Union army's extraordinary sacrifice at Gettysburg, U.S. president Abraham Lincoln visited the Gettysburg Battlefield later that year, where he delivered his historic 271-word Gettysburg Address, widely considered one the most famous speeches in American history.

In the late 19th and 20th centuries, Pennsylvania's manufacturing-based economy blossomed, contributing to the development of much of the nation's early infrastructure, including key bridges, skyscrapers, and much of the military hardware that allowed the U.S. to prevail in World War I, World War II, and the Cold War. In the late 1970s and early 1980s, however, the nation's heavy manufacturing base struggled as the two largest steel manufacturers for most of the 20th century, both based in Pennsylvania, U.S. Steel in Pittsburgh and Bethlehem Steel in Bethlehem, confronted unprecedented challenges from foreign manufacturers and other threats to their historic industry leadership; Bethlehem Steel subsequently ceased operations in 1982 and was liquidated in 2003.

Pennsylvania's geography is highly diverse. The Appalachian Mountains run through the center of the state, the Allegheny and Pocono mountains span much of Northeastern Pennsylvania, and close to 60% of the state is forested. Although it has no ocean shoreline, it has 140 miles (225 km) of waterfront along Lake Erie and the tidal Delaware River.

Pennsylvania is the fifth-most populous state in the United States, with over 13 million residents as of the 2020 United States census, its highest decennial census count ever. The state is the 33rd-largest by area and has the ninth-highest population density among all states. The largest metropolitan statistical area is the southeastern Delaware Valley, including and surrounding Philadelphia, the state's largest and nation's sixth-most populous city. The second-largest metropolitan area, Greater Pittsburgh, is centered in and around Pittsburgh, the state's second-largest city. The state's subsequent five most populous cities are Allentown, Reading, Erie, Scranton, and Bethlehem.