Springfield is the most populous city in and the seat of Hampden County, Massachusetts, United States. Springfield sits on the eastern bank of the Connecticut River near its confluence with three rivers: the western Westfield River, the eastern Chicopee River, and the eastern Mill River. At the 2020 census, the city's population was 155,929, making it the 3rd most populous city in the U.S. state of Massachusetts and the 4th most populous city in New England after Boston, Worcester, and Providence. Metropolitan Springfield, as one of two metropolitan areas in Massachusetts, had a population of 699,162 in 2020.

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.