West Middlesex is a borough along the Shenango River in southwestern Mercer County, Pennsylvania, United States. The population was 863 at the 2010 Census. It is part of the Youngstown-Warren-Boardman, OH-PA Metropolitan Statistical Area.
West Middlesex is located at 41°10′25″N 80°27′25″W (41.173630, -80.456874).
According to the United States Census Bureau, the borough has a total area of 0.9 square miles (2.3 km2). The Shenango River runs through West Middlesex, making it part of the Shenango Valley.
West Middlesex is one of the oldest localities in Mercer County. Samuel Byers, Andrew Wylie, William Bell, Richard Vanfleet and several others arrived in the vicinity in the late 18th century. In 1787, James Gibson received a plot of land west of the Shenango River for military services. Shortly thereafter, ownership of that land passed to Jacob Edeburn. In 1818, Jacob's son William Edeburn built a log grist mill on part of the land.