Jacksonville (US: /ˈdʒæksənˌvɪl/ ^ⓘ JAK-sən-vil) is the most populous city proper in the U.S. state of Florida, located on the Atlantic coast of northeastern Florida. It is the county seat of Duval County, with which the city consolidated in 1968. It is the tenth-most populous U.S. city and the largest city in the Southeast, with a population of 949,611 at the 2020 census (estimated at over 1 million in 2024). The Jacksonville metropolitan area, at over 1.76 million residents, is the fourth-largest metropolitan area in Florida and 38th-largest in the United States. City-county consolidation greatly increased Jacksonville's official population and extended its boundaries, placing most of Duval County's population within the new municipal limits; Jacksonville grew to 900 square miles (2,300 km²). It is the largest city by total area, land and water, in the contiguous United States.

Jacksonville straddles the St. Johns River in the First Coast region of northeastern Florida, about 12 miles (19 kilometers) south of the Georgia state line (25 mi or 40 km to the urban core/downtown) and 350 miles (560 km) north of Miami. The Jacksonville Beaches communities are along the adjacent Atlantic coast. The area was originally inhabited by the Timucua people, and in 1564 was the site of the French colony of Fort Caroline, one of the earliest European settlements in what is now the continental United States. Under British rule, a settlement grew at the narrow point in the river where cattle crossed, known as Wacca Pilatka to the Seminole and the Cow Ford to the British. A platted town was established there in 1822, a year after the United States gained Spanish Florida; it was named after Andrew Jackson, the first military governor of the Florida Territory and seventh President of the United States.

Harbor improvements since the late 19th century have made Jacksonville a major military and civilian deep-water port. Its riverine location facilitates Naval Station Mayport, Naval Air Station Jacksonville, the U.S. Marine Corps Blount Island Command, and the Port of Jacksonville (JAXPORT), Florida's largest seaport by volume. Jacksonville's military bases and the nearby Naval Submarine Base Kings Bay form the third largest military presence in the United States. Significant factors in the local economy include services such as banking, insurance, healthcare and logistics. As with much of Florida, tourism is important to the Jacksonville area, particularly tourism related to golf with the PGA Tour headquarters located in nearby Ponte Vedra Beach. People from Jacksonville are known as Jacksonvillians and, informally, as Jaxsons or Jaxons (both derived from Jax, the shortened nickname for the city).

The area of the modern city of Jacksonville has been inhabited for thousands of years. On Black Hammock Island in the national Timucuan Ecological and Historic Preserve, a University of North Florida team discovered some of the oldest remnants of pottery in the United States, dating to 2500 BCE.

In the 16th century, the beginning of the historical era, the region was inhabited by the Mocama, a coastal subgroup of the Timucua people. At the time of contact with Europeans, all Mocama villages in present-day Jacksonville were part of the powerful chiefdom known as the Saturiwa, centered around the mouth of the St. Johns River. One early French map shows a village called Ossachite at the site of what is now downtown Jacksonville; this may be the earliest recorded name for that area.

In 1562, French Huguenot explorer Jean Ribault charted the St. Johns River, calling it the River of May because that was the month of his discovery. Ribault erected a stone column at his landing site near the river's mouth, claiming the newly discovered land for France. In 1564, René Goulaine de Laudonnière established the first European settlement on the St. Johns River, Fort Caroline, near the main village of the Saturiwa.

Philip II of Spain ordered Pedro Menéndez de Avilés to protect the interests of Spain by attacking the French at Fort Caroline. On September 20, 1565, a Spanish force from the nearby Spanish settlement of St. Augustine attacked Fort Caroline, and killed nearly all the French soldiers defending it. The Spanish renamed the fort as San Mateo and, following the expulsion of the French, St. Augustine became the most important European settlement in Florida. The location of Fort Caroline is subject to debate, but a reconstruction of the fort was established in 1964 along the St. Johns River.

Spain ceded Florida to the British in 1763 as part of the Treaty of Paris in the aftermath of the Seven Years' War (known as the French and Indian War on the North American front). The British soon constructed the King's Road connecting St. Augustine to Georgia. The road crossed the St. Johns River at a narrow point, which the Seminole called Wacca Pilatka and the British called the Cow Ford; these names reflected the use of the ford for moving cattle across the river there.