Lehigh County (/ˈliːhaɪ/; Pennsylvania Dutch: Lechaa Kaundi) is a county in the Commonwealth of Pennsylvania. As of the 2020 census, the county's population was 374,557. Its county seat is Allentown, the state's third-largest city after Philadelphia and Pittsburgh.

Along with Northampton County to its east, the two counties combine to form the Lehigh Valley region of eastern Pennsylvania, the third-largest metropolitan area of Pennsylvania with a population of 861,889 as of 2020. Lehigh County is one of the fastest-growing counties in Pennsylvania and the more highly populated of the two counties. Both counties are part of the Philadelphia television market, the fourth-largest television market in the nation.

The county is named for the Lehigh River, a 109-mile-long (175 km) tributary of the Delaware River, which flows through Lehigh County. During the Industrial Revolution, the Lehigh River served a vital role in the nation's development by offering one of the first transportation and trading routes for mining products, including anthracite, iron, manganese, limestone, and ultimately manufactured steel products.

Lehigh County falls geographically between two Pennsylvania Appalachian mountain ridges, Blue Mountain to the county's north and South Mountain to its south. Lehigh County borders Montgomery County to its south, Bucks County to its southeast, Northampton County to its east, Carbon County to its north, Schuylkill County to its northwest, and Berks County to its southwest.

The county is located 64 miles (103 km) northwest of Philadelphia and 93 miles (150 km) west of New York City.

Lehigh County was first settled around 1730 and was formed in 1812 when Northampton County was divided into two counties. The county is named after the Lehigh River, a 109-mile-long (175 km) river that runs through the county and whose name is derived from the Lenape Indian term Lechauweki or Lechauwekink, meaning "where there are forks." Shelter House, constructed in Emmaus in 1734 by Pennsylvania German settlers, is the oldest continuously occupied structure in both Lehigh County and Lehigh Valley and among the oldest still-standing building structures in the U.S. state of Pennsylvania.

Some of the first resistance to British colonialism, which led ultimately to the American Revolutionary War, began in present-day Lehigh County. On December 21, 1774, patriots in the area formed one of the colonies' first Committee of Observations. Following the Declaration of Independence, patriot militas pressured Tories out of Allentown and the surrounding area, and the colonial government in the area began to break down.

After Washington and the Continental Army were defeated at the Battle of Brandywine on September 11, 1777, the revolutionary capital of Philadelphia was left defenseless and Pennsylvania's Supreme Executive Council ordered that eleven Philadelphia bells, including the Liberty Bell (then known as the State House Bell), be taken down and moved to present day Allentown (then called Northampton Towne) and hidden in the basement of Zion Reformed Church on present day West Hamilton Street to protect them from being melted down by the British Army for use as munitions.