Hampden County is a non-governmental county located in the Pioneer Valley of the U.S. state of Massachusetts, in the United States. As of the 2020 census, Hampden County's population was 465,825. Its traditional county seat is Springfield, the Connecticut River Valley's largest city, and economic and cultural capital; with an estimated population of 154,758, approximately one-third of Hampden County residents live in Springfield.

The Pioneer Valley is the colloquial and promotional name for the portion of the Connecticut River Valley that is in Massachusetts in the United States. It is generally taken to comprise the three counties of Hampden, Hampshire, and Franklin. The lower Pioneer Valley corresponds to the Springfield, Massachusetts metropolitan area, the region's urban center, and the seat of Hampden County. The upper Pioneer Valley region includes the smaller cities of Northampton and Greenfield, the county seats of Hampshire and Franklin counties, respectively.