Tredyffrin Township ( (town) + Dyffryn (valley)) is a township located in eastern Chester County, Pennsylvania, United States. The population was 29,332 at the 2010 census. Originally settled in the late 17th century, Tredyffrin is bounded by Delaware and Montgomery counties. It includes on its northern boundary a small part of Valley Forge National Historical Park, where George Washington encamped during the American War of Independence. Tredyffrin and the entire Great Valley region also have many limestone deposits.
Unincorporated communities within Tredyffrin Township include Chesterbrook, Strafford, a portion of Paoli, and a portion of Wayne. Tredyffrin is bordered by Easttown, East Whiteland, Willistown, Charlestown, Upper Merion, Radnor and Schuylkill townships.
On 13 March 1684, William Penn decided to set aside a portion of Pennsylvania for the Welsh to settle in. This area, known as the "Welsh Tract", was surveyed in 1684 and included several modern-day townships, including Tredyffrin. Although the Welsh Tract was originally meant to be a self-ruling municipality, it was divided between Chester County and Philadelphia when that county was created in 1685, and the Welsh subsequently submitted to the authority of Chester County. In 1707, Tredyffrin was incorporated as a township. This name comes from the Welsh tref, which means "town", joined to dyffryn, which means "wide, cultivated valley". Tredyffrin was quick to develop into a thriving township. In 1710, the first mill in Tredyffrin was started by Thomas Jerman on what is now North Valley Road. Swedesford Road, one of the first roads in the township, was created by 1718.
As the population of the township grew, with 30 resident landowners in 1722 and 83 in 1774, churches sprung up to meet the demand by religious residents. In 1710, the Baptist minister Hugh Davis moved to the Great Valley from Britain and, there being several Baptist families in this area, the Great Valley Baptist Church was formed on 22 April 1711. The church originally met in Radnor. However, there were enough members of this Baptist church that by 1722, a log building was erected in Tredyffrin to serve the congregation. In 1714, the Great Valley Presbyterian Church was started in Tredyffrin. It was the first Presbyterian church in Chester County. A church building was built in 1720 and was used for 73 years. Also in 1714, the Quaker Haverford Monthly Meeting (which was established in April 1684) voted to allow its members that lived in the Great Valley to hold a meeting every other month in Tredyffrin, which was called the Valley Meeting. There was also an Anglican church, built in 1700.
As Tredyffrin includes part of Valley Forge National Historical Park, there are many Revolutionary War-related sites in the township, especially the park and houses where various generals were quartered.
Development in the township was sparked in the 19th century by construction and advertisement of the "Main Line" railroad service, which terminated in Paoli, which straddles the western border of the township.
Tredyffrin-Easttown school district was unusual for the area in being integrated during the early part of the century. In the 1932, the district planned to segregate after building a new school, and African-Americans in the township boycotted the school system for two years in protest (the "School Fight"). In 1934 a negotiated settlement was reached whereby the schools remained integrated, and helped kill school segregation in Pennsylvania.