Berkshire County is the westernmost county in the U.S. state of Massachusetts. As of the 2020 census, the population was 129,026. Its largest city and traditional county seat is Pittsfield. The county was founded in 1761. The Berkshire Hills are centered on Berkshire County. Residents are known as Berkshirites. It exists today only as a historical geographic region, and has no county government, with the exception of the retirement board for former county workers, and certain offices such as the sheriff and registry of deeds.

Western Massachusetts, known colloquially as "western Mass," is a region in Massachusetts, one of the six U.S. states that make up the New England region of the United States. Western Massachusetts has diverse topography; 22 colleges and universities including UMass in Amherst, MA, with approximately 100,000 students; and such institutions as Tanglewood, the Springfield Armory, and Jacob's Pillow.