Fairfield County is a county in the southwestern corner of the U.S. state of Connecticut. It is the most populous county in the state and was also its fastest-growing from 2010 to 2020. As of the 2020 census, the county's population was 957,419, representing 26.6% of Connecticut's overall population. The closest to the center of the New York metropolitan area, the county contains four of the state's seven largest cities—Bridgeport (first), Stamford (second), Norwalk (sixth) and Danbury (seventh)—whose combined population of 433,368 is nearly half the county's total population.

The United States Office of Management and Budget has designated Fairfield County as the Bridgeport–Stamford–Norwalk metropolitan statistical area. The United States Census Bureau ranked the metropolitan area as the 59th most populous metropolitan statistical area of the United States in 2019. The U.S. Office of Management and Budget has further designated the metropolitan statistical area as a component of the more extensive New York–Newark–Bridgeport, NY–NJ–CT–PA combined statistical area, the most populous combined statistical area and primary statistical area of the United States.

As is the case with all eight of Connecticut's counties, there is no county government and no county seat. As an area, it is only a geographical point of reference. In Connecticut, the cities and towns are responsible for all local governmental activities including fire and rescue, schools, and snow removal; in a few cases, neighboring towns will share certain resources. The last county seat was Bridgeport, which had served this role from 1853 until 1960. On June 6, 2022, the U.S. Census Bureau formally recognized Connecticut's nine councils of governments as county equivalents instead of the state's eight counties. Connecticut's eight historical counties continue to exist in name only, and are no longer considered for statistical purposes.

Fairfield County's Gold Coast helped rank it sixth in the U.S. in per-capita personal income by the Bureau of Economic Analysis in 2005, contributing substantially to Connecticut being one of the most affluent states in the U.S. Other communities are more densely populated and economically diverse than the affluent areas for which the county is better known.

Fairfield County was the home of many Native American tribes prior to the Europeans' arrival. People of the Schaghticoke tribe lived in the area of present-day New Fairfield and Sherman. From east to west the Wappinger sachemships included the Paugussetts, Tankiteke, and the Siwanoy. There were also Paquioque and Potatuck inhabitants of Fairfield County.

The Dutch explorer Adriaen Block explored coastal Connecticut in the Spring and early Summer of 1614 in the North America-built vessel Onrust. The first European settlers of the county, however, were Puritans and Congregationalists from England. Roger Ludlow (1590–1664), one of the founders of the Colony of Connecticut, helped to purchase and charter the towns of Fairfield (1639) and Norwalk (purchased 1640, chartered as a town in 1651). Ludlow is credited as having chosen the name Fairfield. Fairfield is a descriptive name referring to the beauty of its fields. The town of Stratford was settled in 1639 as well by Adam Blakeman (1596–1665). William Beardsley (1605–1661) was also one of the first settlers of Stratford in 1639.

Fairfield County was established by an act of the Connecticut General Court in Hartford along with Hartford County, New Haven County, and New London County; which were the first four Connecticut counties, on May 10, 1666. From transcriptions of the Connecticut Colonial Records for that day:

The original Fairfield County consisted of the towns of Rye, Greenwich, Stamford, Norwalk, Fairfield, and Stratford. In 1673, the town of Woodbury was incorporated and added to Fairfield County. In 1683, New York and Connecticut reached a final agreement regarding their common border. This resulted in the cession of the town of Rye and all claims to the Oblong to New York. From the late 17th to early 18th centuries, several new towns were incorporated in western Connecticut and added to Fairfield County, namely Danbury (1687), Ridgefield (1709), Newtown (1711), and New Fairfield (1740). In 1751, Litchfield County was constituted, taking over the town of Woodbury. The final boundary adjustment to Fairfield County occurred in 1788 when the town of Brookfield was incorporated from parts of Newtown, Danbury, and New Milford, with Fairfield County gaining territory from Litchfield County.