Lancaster is a town in Worcester County, Massachusetts, United States. Incorporated in 1653, Lancaster is the oldest town in Worcester County. As of the 2020 census, the town population was 8,441. It contains the village of South Lancaster.

In 1643 Lancaster was first settled as "Nashaway" (named after the local Nashaway Native American tribe) by a group of colonists known as the Nashaway Company who may have initially been interested in iron deposits in the area. Several of the company were blacksmiths or gunsmiths, including, Herman Garrett, and as early as 1653 a settler, George Adams, was whipped for selling guns and alcohol to the Indians in the area. The town was officially incorporated and renamed "Lancaster on the Nashua" in 1653. Prominent Massachusetts military leader Simon Willard served as an advisor to the company and eventually settled in Lancaster for a period, and provided guns to the local tribe by order of the Massachusetts General Court.

Supporters of Lancaster's founder, John Prescott, born in 1604 (great grandfather of Bunker Hill leader William Prescott), wished to name the new settlement Prescott, but the Massachusetts General Court considered such a request from a common freeman presumptuous, given that at that time, not even a governor had held the honor of naming a town after himself. Instead, they decided to use Lancaster, the name of Prescott's home town in England.

Until it was cut down in 1989 due to safety concerns, Lancaster boasted the largest oak tree in the state, called the Beaman Oak, named after settler Gamaliel Beaman (1623–1677).

Lancaster boasts being the official "mothertown" to all of Eastern central Massachusetts. Towns such as Harvard, Bolton, Leominster, Clinton, Berlin, Sterling, and part of West Boylston were all formed from territory of the original boundaries of Lancaster.

During Metacom's War, in February 1676 (1675 old style calendar), Mary Rowlandson was taken captive by Native Americans during the Lancaster raid. Rowlandson later wrote a book about her experiences titled A Narrative of the Captivity and Restoration of Mrs. Mary Rowlandson. In 2000, Lancaster Elementary School changed its name to Mary Rowlandson Elementary School.

According to the United States Census Bureau, the town has a total area of 28.2 square miles (73 km²), of which 27.7 square miles (72 km²) is land and 0.5 square miles (1.3 km²), or 1.84%, is water.^{[citation needed]}

Lancaster is bordered by Lunenburg and Shirley to the north, Harvard to the northeast, Bolton to the southeast, Clinton to the south, Sterling to the southwest, and Leominster to the northwest.^{[citation needed]}