William Penn Charter School (commonly known as Penn Charter or simply PC) is a private school in Philadelphia, Pennsylvania. It was founded in 1689 at the urging of William Penn as the "Public Grammar School" and chartered in 1689 to be operated by the "Overseers of the public School, founded by Charter in the town and county of Philadelphia" in Pennsylvania. It is the oldest Quaker school in the world, the oldest elementary school in Pennsylvania, and the fifth oldest elementary school in the United States following The Collegiate School ("claimed" 1628), Boston Latin School (1635), Hartford Public High School (1638), and Roxbury Latin (1645).

Penn Charter is among the first schools in the United States to offer education to all religions (1689), financial aid (1701), matriculation to girls (1754), and education to all races (1770). The "Charter" in the school's name does not, as might be assumed, mean that it is a modern "charter school". Rather, it is a reference to the historic document signed by William Penn to establish the first Quaker school in America. Originally located on the east side of Fourth Street below Chestnut, the school officially consolidated in 1874 as an all-boys College-preparatory school at 12th and Market Streets. Penn Charter moved to its current forty-seven-acre East Falls campus in 1925. In 1980, the school became fully co-educational by allowing girls to continue past the second grade, and graduated the first co-ed senior class in 1992.

While the school is not under the care of a formal monthly Meeting, in keeping with the school's Quaker heritage, the Overseers, a board of 21 trustees established by William Penn, still governs the school affairs through Quaker consensus. Jeffrey Reinhold is the current clerk of the Overseers. All students attend a weekly meeting for worship. Faculty meetings and all-school assemblies, and some classes begin with a moment of silence.

Service-learning is integral to the school and incorporated into the pre-K to 12 curriculum. The school's Center for Public Purpose engages students in service and community-based work by addressing Philadelphia's most pressing social issues, particularly education, food insecurity, and poverty. To earn activity credit, many Upper School students complete 40 hours of community service a year; a van carrying students leaves campus after school every day to perform service at various locations throughout the Philadelphia area.

Color Day, celebrated on the Friday before Memorial Day, is a tradition in which two teams sporting the school's colors, blue and yellow, compete against each other in playful contests, concluding with a 12th-grade rope pull.

The school's Senior Stairs are a central stairway that only current seniors, faculty, and alumni can use during school hours.

A Penn Charter graduate is known as an "OPC." The honorific "OPC 1689" is bestowed rarely by the Overseers upon significant faculty and staff in recognition of their service to Old Penn Charter.

The school newspaper, The Mirror, is the oldest secondary school student newspaper in the United States, having been published since 1777.