Marion County is a county located in the North Central region of the U.S. state of Florida. As of the 2020 United States census, the population was 375,908. Its county seat is Ocala. Marion County comprises the Ocala, Florida Metropolitan Statistical Area. It includes part of Ocala National Forest, which also extends into three other counties.

Evidence of ancient indigenous cultures has been found in Marion County, as well as of the earliest encounter between European explorers and historic indigenous peoples. In 1976, an archaeological investigation found ancient artifacts in Marion County that appear to be the oldest in mainland United States. Excavations at an ancient stone quarry (on the Container Corporation of America site (8Mf154) in Marion County) yielded "crude stone implements". Thousands of pieces of chert were found at the site. These showed signs of extensive wear and were found in deposits below those holding Paleo-Indian artifacts. Thermoluminescence dating and weathering analysis independently gave dates of 26,000 to 28,000 Years Before Present (YBP) for the production of these artifacts, prior to Clovis points. The findings suggested human habitation in this area much earlier than documented by other evidence. Barbara Purdy had bipoint evidence from the CCA site, which also predated Clovis points, which she reported in a 2008 paper.

The county seat of Ocala, Florida, is named for a Timucuan village visited and recorded by the Spanish Hernando de Soto expedition in the sixteenth century. During the colonial period, Spain and Great Britain traded control of this area.

After acquisition of the Florida territory by the United States in the 1820s, Marion County was created in 1844 from portions of Alachua, Mosquito (Orange), and Hillsborough counties. Until 1853, Marion County included most of what are now Lake and Sumter counties. In 1849, Putnam County was created and took the northeast portion of Marion. Levy County's creation took some of the western portion of Marion in 1877, near the end of the Reconstruction era.

Marion County is named after General Francis Marion of South Carolina, a guerrilla fighter and hero of the American Revolutionary War who was known as the "Swamp Fox". Numerous early settlers of this area were natives of South Carolina and likely picked their local hero as the county's namesake. The Act creating the county of Marion of the Territory of Florida was signed on March 14, 1844, by the territorial governor, R. K. Call. The county motto is "Kingdom of the Sun."

Marion County was a hotbed of secessionist activity. Organizers petitioned the state to host the Florida Secession Convention and break away from the United States with other Southern states. The area had many plantations and more than half of the population was enslaved African Americans.

Several Confederate Army units were organized by plantation owners, including the Marion Rifle Guards, organized May 2, 1861, by Captain W.L. Fletcher, county treasurer. It served as part of the Fourth Florida Infantry under Col. Edward Hopkins. Other company's included the Hammock Guards, commanded by Captain J.S. Hopkins; Marion Dragoons, a cavalry unit organized and financed by William A. Owens, Marion Hornets, commanded by Captain Wade Eichelberger, and the Marion Light Artillery, commanded at one time by John Marshall Martin. The Civil War and its aftermath disrupted local society and strained the economy. The population declined in its wake as many African Americans moved to towns or cities for better opportunities.

During the post-Reconstruction period, there was considerable racial violence in Marion County. Locals lynched between 19 and 26 African Americans in the county from the 1880s to 1935.