Jacksonville (US: JAK-sən-vihl) is the most populous city proper in the U.S. state of Florida, located on the Atlantic coast of northeastern Florida. It is the seat of Duval County, with which the City of Jacksonville consolidated in 1968. It was the largest city by area in the contiguous United States as of 2020, and became the 10th largest city by population in 2023.

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).

As of July 2022, Jacksonville's population was 971,319, while the population of Duval County was about 1 million. After consolidation, Jacksonville became the most populous city in Florida and the Southeastern United States, and the largest in the South outside the state of Texas. With a population of 1,733,937, the Jacksonville metropolitan area ranks as Florida's fourth-largest metropolitan region. The metropolitan area consists of Clay County, St. Johns County, Nassau County, and Baker County.

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 Florida from Spain; 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).