Billerica is a town in Middlesex County, Massachusetts, United States. The population was 42,119 according to the 2020 census. It takes its name from the town of Billericay in Essex, England.

Middlesex County is a county located in the Commonwealth of Massachusetts, United States. As of the 2020 census, the population was 1,632,002, making it the most populous county in both Massachusetts and New England and the 22nd most populous county in the United States. Middlesex County is one of two U.S. counties to be amongst the top 25 counties with the highest household income and the 25 most populated counties. It is included in the Census Bureau's Boston–Cambridge–Newton, MA–NH Metropolitan Statistical Area. As part of the 2020 United States census, the Commonwealth's mean center of population for that year was geo-centered in Middlesex County, in the town of Natick.