Rittenhouse Square is a public park in Center City Philadelphia, Pennsylvania that is the center of the eponymous Rittenhouse neighborhood. The square is one of the five original open-space parks planned by William Penn and his surveyor Thomas Holme during the late 17th century. Together with Fitler Square, the Rittenhouse neighborhood and the square comprise the Rittenhouse–Fitler Historic District.

Rittenhouse Square is maintained by the non-profit group The Friends of Rittenhouse Square. The square cuts off 19th Street at Walnut Street and also at a half-block above Manning Street. Its boundaries are 18th Street to the east, Walnut Street to the north, and Rittenhouse Square West, a north–south boundary street, and Rittenhouse Square South, an east–west boundary street, making the park approximately two short blocks on each side. Locust Street borders Rittenhouse Square to both its east and west in the middle of the square.

Originally called Southwest Square, Rittenhouse Square was renamed in 1825 after David Rittenhouse, a descendant of the first paper-maker in Philadelphia, the German immigrant William Rittenhouse. William Rittenhouse's original paper-mill site is known as Rittenhousetown, located in the rural setting of Fairmount Park along Paper Mill Run. David Rittenhouse was a clockmaker and friend of the American Revolution, as well as a noted astronomer; a lunar crater is named after him.

In the early 19th century, as the city grew steadily from the Delaware River to the Schuylkill River, Rittenhouse Square became a highly desirable address.^{[citation needed]} James Harper, a merchant and brick manufacturer who had recently retired from the United States Congress, was the first person to build on the square, buying most of the north frontage, erecting a stately townhouse for himself at 1811 Walnut Street (c. 1840). Having thus set the patrician residential tone that would subsequently define the Square, he divided the rest of the land into generously proportioned building lots and sold them. Sold after the congressman's death, the Harper house became the home of the exclusive Rittenhouse Club, which added the present facade in c. 1901.

From 1876 to 1929, Rittenhouse Square was home to several wealthy families including Pennsylvania Railroad president Alexander Cassatt, real estate entrepreneur William Weightman III, department store founder John Wanamaker, Philadelphia planning commission director Edmund Bacon and his son, actor Kevin Bacon, as well as others.^{[citation needed]}

Elegant churches and clubs were constructed by John Notman and Frank Furness. In 1913, French architect Paul Philippe Cret redesigned parts of the Square to resemble Paris and the French gardens, adding classical entryways and stone additions to railings, pools, and fountains.

After World War II, Rittenhouse Square's Victorian mansions began to be replaced with high-rise residential and office buildings such as Claridge and Savoy. Vacant lots were converted to apartments and hotels. Still, some prominent Italianate and Art Deco buildings remain, and Rittenhouse Square has changed the least out of the city's initial squares. Journalist and author Jane Jacobs wrote about two main ideas in Cret's redesign: intricacy and centering.

In the mid-20th century, the park became known as a safe area for gays and lesbians to meet in Center City.