Genesee County (/ˈdʒɛnəsi/ JEN-ə-see) is a county in the U.S. state of Michigan. As of the 2020 Census, the population was 406,211, and was estimated to be 402,279 in 2024, making it the fifth-most populous county in Michigan, and the most populous in Mid-Michigan. The county seat and largest city is Flint (birthplace of General Motors). Genesee County consists of 33 cities, townships, and villages. It is considered to be a part of Mid-Michigan.

The county was named after Genesee County, New York, which in turn comes from the Seneca word Gen-nis'-hee-yo, meaning "Beautiful Valley". Genesee County comprises the Flint, MI Metropolitan statistical area. A major attraction for visitors is Crossroads Village, a living history village north of Flint.

Genesee County is noted for having had the fossil of an ancient whale known as Balaenoptera Lacepede unearthed in Thetford Township during quarry work and estimated at 11,000 years old.

Genesee County was created on March 28, 1835, from territory taken from Lapeer, Shiawassee and Saginaw Counties. The county was attached to Oakland County for judicial purposes. The only township organized at the time was Grand Blanc, formed in 1833 consisting of additional township areas of the present Fenton, Mundy, Flint, Mount Morris, Genesee, Burton, Atlas and Davison townships. (Atlas and Davision township survey areas were then in Lapeer County.) Flint Township followed in formation by the legislature on March 2, 1836, splitting away from Grand Blanc with the township areas of Burton, Genesee and Mount Morris plus additional township areas of the present day Clayton, Montrose, Flushing, Thetford and Vienna. On April 4, 1836, the county was fully organized. Organized on July 26, 1836, Argentine Township included two township survey areas taking Fenton away from Grand Blanc Township plus the current Argentine area west of Fenton.

In the organizing act of March 11, 1837, two townships (Mundy and Vienna) were organized in the county. Mundy had an additional township area to the west. Vienna consisted of the northern tier of three township areas in the county at the time taking two areas from Flint Township. With this act just a week after Michigan's admission as a state, the county was fully covered with 5 township governments.

The townships of Genesee, Fenton, and Flushing were formed on March 6, 1838. Genesee and Flushing were split off from Flint Township. Fenton was split off from Argentine Township. Genesee included half the survey area to the west. While Flushing included the other half of that survey area (township 8 North range 6 East), the full township area further west (township 8 North range 5 East) then another survey area and half the row below (township 7 North range 5,6 East). Thus Flint Township consisted of one and a half township survey areas.

Fifteen northerly sections of survey area township 7 north range No. 7 east in Flint Township plus southerly sections of Genesee Township were set off by the Michigan Legislature as Kearsley Township in 1839. On March 7, 1843, the legislature abolished this township returning the sections back to Flint and Genesee townships. Thus Flint Township consisted of approximately one and a half township survey areas.

Two new townships were formed by the state on March 9, 1842. Thetford was the eastmost survey area split off from Vienna. While Gaines was formed with the western township survey of Mundy.