Market Street, originally known as High Street, is a major east–west highway and street in Philadelphia, Pennsylvania, United States. The street is signed as Pennsylvania Route 3 between 38th Street (U.S. Route 13) and 15th Street (PA 611). A short portion of the road continues west from Cobbs Creek Parkway (63rd Street) to Delaware County, adjacent to Philadelphia. The street also serves as the dividing line for the "north" and "south" sides of the city. All north-south addresses in the city start at zero at Market Street.

High Street was the familiar name of the principal street in nearly every English town at the time Philadelphia was founded. But if Philadelphia was indebted to England for the name of High Street, nearly every American town is, in turn, indebted to Philadelphia for its Market Street. Long before the city was laid out or settled, Philadelphia's founder, William Penn, had planned that markets would be held regularly on the 100-foot (30 m) wide High Street.

The city's first market stalls were situated in the center of the thoroughfare starting at Front Street and proceeding west eventually to 8th Street. The stalls soon became covered and were not taken down as planned. Later, additional covered sheds appeared west of Center Square as the city expanded westward. The street began to be called Market Street around 1800. The road's new name was made official by an ordinance in 1858, coincidentally just a year before the market sheds were ordered removed.

Market Street has been called the most historic highway in the United States because of the various historic sites along its eastern section. Many of Benjamin Franklin's activities were centered along Market Street. His house was located near the intersection of Fourth Street, and he may have performed his famous kite-flying experiment near Third and Market Streets.

Over a period of two weeks in June 1776, Thomas Jefferson wrote the Declaration of Independence in a boarding house, known as the Graff or Declaration House, once located at 700 Market Street at the corner of 7th and Market streets.

The mansion of Robert Morris, financier of the American Revolution, was located near 6th and Market Streets. This house, known as the President's House, was used by George Washington and John Adams as their residence during their terms as president prior to the completion of the White House in 1800. The house was located on the site of the northern part of the present-day Liberty Bell Center. Around 1795, Theophilus Cazenove lived on Market Street. Several important finance and publishing firsts also occurred along Market Street between Second and Fourth Streets during the 18th century. Market Street is still one of the principal locations of business and commerce in Philadelphia.

Market Street runs from Millbourne to Front Street in Center City, Philadelphia. At Front Street, a bridge over Interstate 95 brings traffic from Penn's Landing, on the western bank of the Delaware River, onto westbound Market Street. Market Street runs one way, eastbound, between 20th Street and 15th Street, with westbound traffic diverted onto JFK Boulevard.

As of 2023, the entire length of Market Street is part of Philadelphia's High Injury Network, the small fraction of city streets on which the majority of traffic deaths and serious injuries occur. Market Street includes a two-way protected bike lane between 15th and 20th Streets and painted bike lanes between 34th and 63rd Streets.