Jefferson County is a county located in the U.S. state of Mississippi; its western border is formed by the Mississippi River. As of the 2020 census, the population was 7,260, making it the fourth-least populous county in Mississippi. Its first county seat was located at Old Greenville until 1825, which no longer exists, before moving to Fayette. The county is named for U.S. President Thomas Jefferson.

The Territory of Mississippi was an organized incorporated territory of the United States that was created under an organic act signed into law by President John Adams on April 7, 1798. It was dissolved on December 10, 1817, when the western half of the territory was admitted to the Union as the State of Mississippi. The eastern half was redesignated as the Alabama Territory; it was admitted to the Union as the State of Alabama on December 14, 1819. The Chattahoochee River played a significant role in the definition of the territory's borders. The population increased in the early 1800s from settlement, with cotton being an important cash crop.