Franklin County is a nongovernmental county in the northwestern part of the U.S. state of Massachusetts. At the 2020 census, the population was 71,029, which made 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.

Franklin County was created on June 24, 1811, from the northern third of Hampshire County. It was named for Benjamin Franklin. Franklin County's government was abolished by the state government in 1997, at the county's request.

Like several other Massachusetts counties, Franklin County exists today only as a geographic region and has no county government. The Franklin County Commission voted itself out of existence, and all former state-mandated county functions were assumed by state agencies in 1997. The sheriff and some other regional officials with specific duties are still elected locally to perform duties within the county region. Counties in Massachusetts and New England generally are historically weak governmental structures. The primary subdivision of the Commonwealth is the municipal town. Communities are permitted to form regional compacts for sharing services. The municipalities of Franklin County have formed the Franklin Regional Council of Governments. The regional council provides various services on a regional basis, and a majority of the county's towns are members of the Franklin County Solid Waste Management District, which provides municipal waste disposal and recycling services to its members. Public transportation throughout the county and in the North Quabbin area of northwestern Worcester County is provided by the Franklin Regional Transit Authority.

According to the U.S. Census Bureau, the county has a total area of 724.57 square miles (1,876.6 km²), of which 699 square miles (1,810 km²) is land, 25 square miles (65 km²) and (3.5%) is water. Central and southern Franklin County is dominated by the northern end of the Pioneer Valley, with steep hills rising on either side of the Connecticut River.

The high point of Franklin County is Crum Hill, 2,841 feet (866 m), located in the town of Monroe.

The climate in Franklin County is typically cool temperate. The area is also somewhat maritime, with relatively high year-round precipitation. Summers are warm and humid with frequent evening storms, and winters are cool to cold with frequent snow and subfreezing (below 31 °F) temperatures.

Various Department of Conservation & Recreation properties.

At the 2000 census there were 71,535 people, 29,466 households, and 18,416 families in the county. The population density was 102 inhabitants per square mile (39/km²). There were 31,939 housing units at an average density of 46 per square mile (18/km²). The racial makeup of the county was 95.40% White, 0.89% Black or African American, 0.29% Native American, 1.04% Asian, 0.03% Pacific Islander, 0.75% from other races, and 1.61% from two or more races. 1.99%. were Hispanic or Latino of any race. 16.2% were of English, 12.2% Irish, 12.0% Polish, 10.2% French, 7.0% French Canadian, 6.7% German, 6.1% Italian and 6.0% American ancestry according to Census 2000. Most of those claiming to be of "American" ancestry are actually of English descent, but have family that has been in the country for so long, in many cases since the early seventeenth century that they choose to identify simply as "American". 94.5% spoke English and 1.8% Spanish as their first language.