Louisville is the most populous city in the Commonwealth of Kentucky, sixth-most populous city in the Southeast, and the 27th-most-populous city in the United States. By land area, it is the country's 24th-largest city; however, by population density, it is the 265th most dense city. Louisville is the historical county seat and, since 2003, the nominal seat of Jefferson County, on the Indiana border.

Since 2003, Louisville and Jefferson County have shared the same borders following a city-county merger. The consolidated government is officially called the Louisville/Jefferson County Metro Government, commonly known as Louisville Metro. The term "Jefferson County" is still used in some contexts, especially for incorporated cities outside the "balance" area that defines Louisville proper. The total population of the consolidated area was 782,969 at the 2020 census, while the balance area (excluding other incorporated cities) had a population of 633,045 and is often cited in national statistics. The Louisville metropolitan area, which includes 12 surrounding counties in Kentucky and Southern Indiana, has 1.39 million residents and is the 43rd-largest metropolitan area in the U.S.

Named after King Louis XVI of France, Louisville was founded in 1778 by George Rogers Clark, making it one of the oldest cities west of the Appalachians. With the nearby Falls of the Ohio as the only major obstruction to river traffic between the upper Ohio River and the Gulf of Mexico, the settlement first grew as a portage site. It was the founding city of the Louisville and Nashville Railroad, which grew into a 6,000-mile (9,700 km) system across 13 states. Today, the city is known as the home of boxer Muhammad Ali, the Kentucky Derby, Kentucky Fried Chicken, the University of Louisville and its Cardinals, Louisville Slugger baseball bats, and Fortune 500 company Humana. Louisville Muhammad Ali International Airport, the city's main commercial airport, hosts UPS's worldwide hub.