Franklin County is a nongovernmental county located in the northwestern part of the U.S. state of Massachusetts. As of the 2020 census, the population was 71,029, which makes it the least-populous county on the Massachusetts mainland, and the third-least populous county in the state. Its traditional county seat and most populous city is Greenfield. Its largest town by area is New Salem. Franklin County comprises the Greenfield Town, MA Micropolitan Statistical Area, which is included in the Springfield-Greenfield Town, MA Combined Statistical Area.

Greenfield is the only city in, and the seat of, Franklin County, Massachusetts, United States. Greenfield was first settled in 1686. The population was 17,768 at the 2020 census. Greenfield is home to Greenfield Community College, the Pioneer Valley Symphony Orchestra, and the Franklin County Fair. The city has a Main Street Historic District containing fine examples of Federal, Greek Revival, and Victorian architecture.