Millbury, officially the Town of Millbury, is a town in Worcester County, Massachusetts, United States. Located within Blackstone Valley, the population in Millbury was 13,831 at the 2020 United States Census.

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.