Alachua County (/əˈlætʃuə/ ^ⓘ ə-LATCH-oo-ə) is a county in the north central portion of the U.S. state of Florida. As of the 2020 census, the population was 278,468. The county seat is Gainesville, the home of the University of Florida.

The first people known to have entered the area of Alachua County were Paleo-Indians, who left artifacts in the Santa Fe River basin before 8000 BC. Artifacts from the Archaic period (8000 - 2000 BC) have been found at several sites in Alachua County. Permanent settlements appeared in what is now Alachua County around 100 AD, as people of the wide-ranging Deptford culture developed the local Cades Pond culture. The Cades Pond culture gave way to the Alachua culture around 600 AD.

The Timucua-speaking Potano tribe lived in the Alachua culture area in the 16th century, when the Spanish entered Florida. The Potano were incorporated by the colonists in the Spanish mission system, but new infectious diseases, rebellion, and raids by tribes backed by the English led to severe population declines. What is now Alachua County had lost much of its indigenous population by the early 18th century.

In the 17th century, Francisco Menéndez Márquez, Royal Treasurer for Spanish Florida, established the La Chua ranch on the northern side of what is now known as Payne's Prairie, on a bluff overlooking the Alachua Sink. Chua may have been the Timucua language word for sinkhole. Lieutenant Diego Peña reported in 1716 that he passed by springs named Aquilachua, Usichua, Usiparachua, and Afanochua while traveling through what is now Suwannee County. In the twentieth-century, anthropologist J. Clarence Simpson assumed the named springs were in fact sinkholes. The Spanish later called the interior of Florida west of the St. Johns River Tierras de la Chua, which became "Alachua Country" in English.

Around 1740, a band of Oconee people led by Ahaya, who was called "Cowkeeper" by the English, settled on what is now Payne's Prairie. Ahaya's band became known as the Alachua Seminole. In 1774, botanist William Bartram visited Ahaya's town, Cuscowilla, near what Bartram called the Alachua Savanna. King Payne, who succeeded Ahaya as chief of the Alachua Seminole, established a new town known as Payne's Town.

In 1812, during the Patriot War of East Florida, an attempt by American adventurers to seize Spanish Florida, a force of more than 100 volunteers from Georgia led by Colonel Daniel Newnan encountered a band of Alachua Seminole led by King Payne near Newnans Lake. After several days of intermittent fighting, Colonel Newnan's force withdrew. King Payne was wounded in the fight and died two months later. The Alachua Seminole then left Payne's Town and moved farther west and south, but other bands of Seminole moved in. A second American expedition in 1813 of U. S. Army troops and militia from Tennessee, led by Lt. Colonel Thomas Adams Smith, found some Seminoles, killing about 20, and burned every Seminole village they could find in the area.

In 1814, a group of more than 100 American settlers moved to a point believed to be near the abandoned Payne's Town (near present-day Micanopy) and declared the establishment of the District of Elotchaway of the Republic of East Florida. The settlement collapsed a few months later after its leader, Colonel Buckner Harris, was killed by Seminole. The remaining settlers returned to Georgia.

In 1817, F. M. Arredondo received the 20-mile square Arredondo Grant in the southern part of what is Alachua County. By the time Florida was formally transferred from Spain to the United States, people from the United States and from Europe were settling in the area. Wanton's Store, near the site of the abandoned King Payne's Town, attracted settlers, primarily from Europe, who founded Micanopy. The 1823 Treaty of Moultrie Creek required the Seminole to move a reservation south of what is now Ocala, and the flow of settlers into the area increased. Many settlers occupied former Seminole towns, such as Hogtown.