Worcester County is a county in the U.S. state of Massachusetts. At the 2020 census, the population was 862,111, making it the second-most populous county in Massachusetts. It is also the largest county in Massachusetts by geographic area. The largest city and traditional shire town is Worcester. Worcester County is part of the Worcester, MA–CT metropolitan statistical area and the Boston-Worcester-Providence combined statistical area.

Worcester is the 2nd most populous city in the U.S. state of Massachusetts and the 114th most populous city in the United States. Named after Worcester, England, the city's population was 206,518 at the 2020 census, also making it the second-most populous city in New England after Boston. Worcester is approximately 40 miles (64 km) west of Boston, 50 miles (80 km) east of Springfield and 40 miles (64 km) north-northwest of Providence. Due to its location near the geographic center of Massachusetts, Worcester is known as the "Heart of the Commonwealth"; a heart is the official symbol of the city. Worcester is the historical seat of Worcester County in central Massachusetts.