Castle Shannon is a borough in Allegheny County, Pennsylvania, United States, and is part of the Pittsburgh Metro Area. The population was 8,316 at the 2010 census.
According to the United States Census Bureau, the borough has a total area of 1.6 square miles (4.1 km2), all of it land. Its average elevation is 1,040 feet (320 m) above sea level.
Castle Shannon has five borders, including Baldwin Township to the north, the Pittsburgh neighborhood of Overbrook to the northeast, Whitehall to the east and southeast, Bethel Park to the south and southwest, and Mt. Lebanon to the west and northwest.
The first families settled Castle Shannon in 1786 in pursuit of farmland and timber. The most prominent farm was owned by David Strawbridge, who named it Castle Shanahan. Over time, the farm would lend its name to the area, as "Shanahan" would evolve into "Shannon". In 1872, the Pittsburgh and Castle Shannon Railroad line was completed, providing a direct link from Pittsburgh to the then-village of Castle Shannon. Development was stimulated by two years of free transportation and lumber transport given to anybody building a home.
In 1877, a second railroad was built from Finleyville through Castle Shannon to the West End neighborhood of Pittsburgh. In 1909, the right of way through the valley containing Castle Shannon was purchased by the Pittsburgh Railroad.