Western Massachusetts is a loosely defined geographical region of the U.S. state of Massachusetts which contains the Berkshires, the Pioneer Valley, and some or all of the Swift River Valley. The region is always considered to include Berkshire, Franklin, Hampshire, and Hampden counties, and the eastern parts of the Quabbin Reservoir watershed are sometimes included. The largest city in Western Massachusetts is Springfield, Massachusetts, the region's economic, historic, and cultural capital.

There are 103 towns and 11 cities in Western Massachusetts, the largest of which is the City of Springfield in the Connecticut River Valley. There are four or five counties in Western Massachusetts, none of which currently serves any administrative purpose: Hampden, Hampshire, Franklin, and Berkshire Counties. Worcester County is sometimes included, though most of it is properly considered Central Massachusetts.

The City of Springfield, Massachusetts, which sits beside the Connecticut River amid the wide Connecticut River Valley, is Western Massachusetts' urban economic and cultural capital. Springfield lies only 24 miles north of Hartford, Connecticut, Connecticut's state capital. The Hartford-Springfield region is known as the Knowledge Corridor due to its 29 colleges and universities and 120,000 college students. Significant Massachusetts towns and cities in the Knowledge Corridor include Greenfield, Northampton, Amherst, Easthampton, Holyoke, Chicopee, West Springfield, East Longmeadow, Longmeadow, Ludlow, Agawam, and Westfield.