Louisiana (French: Louisiane [lwizjan] ; Spanish: Luisiana [lwiˈsjana]; Louisiana Creole: Lwizyàn) is a state in the Deep South and South Central regions of the United States. It borders Texas to the west, Arkansas to the north, and Mississippi to the east. Of the 50 U.S. states, it ranks 20th in land area and the 25th in population, with roughly 4.6 million residents. Reflecting its French heritage, Louisiana is the only U.S. state with political subdivisions termed parishes, which are equivalent to counties, making it one of only two U.S. states not subdivided into counties (the other being Alaska and its boroughs). Baton Rouge is the state's capital, and New Orleans, a French Louisiana region, is its largest city with a population of about 383,000 people. Louisiana has a coastline with the Gulf of Mexico to the south; a large part of its eastern boundary is demarcated by the Mississippi River.

Much of Louisiana's lands were formed from sediment washed down the Mississippi River, leaving enormous deltas and vast areas of coastal marsh and swamp. These contain a rich southern biota, including birds such as ibises and egrets, many species of tree frogs—such as the state-recognized American green tree frog—and fish such as sturgeon and paddlefish. More elevated areas, particularly in the north, contain a wide variety of ecosystems such as tallgrass prairie, longleaf pine forest and wet savannas; these support an exceptionally large number of plant species, including many species of terrestrial orchids and carnivorous plants. Over half the state is forested.

Louisiana is situated at the confluence of the Mississippi river system and the Gulf of Mexico. Its location and biodiversity attracted various indigenous groups thousands of years before Europeans arrived in the 17th century. Louisiana has eighteen Native American tribes—the most of any southern state—of which four are federally recognized and ten are state-recognized. The French claimed the territory in 1682, and it became the political, commercial, and population center of the larger colony of New France. From 1762 to 1801 Louisiana was under Spanish rule, briefly returning to French rule before being sold by Napoleon to the U.S. in 1803. It was admitted to the Union in 1812 as the 18th state. Following statehood, Louisiana saw an influx of settlers from the eastern U.S. as well as immigrants from the West Indies, Germany, and Ireland. It experienced an agricultural boom, particularly in cotton and sugarcane, which were cultivated primarily by slaves from Africa. As a slave state, Louisiana was one of the original seven members of the Confederate States of America during the American Civil War.

Louisiana's unique French heritage is reflected in its toponyms, dialects, customs, demographics, and legal system. Relative to the rest of the southern U.S., Louisiana is multilingual and multicultural, reflecting an admixture of Louisiana French (Cajun, Creole), Spanish, French Canadian, Acadian, Saint-Domingue Creole, Native American, and West African cultures (generally the descendants of slaves stolen in the 18th century); more recent migrants include Filipinos and Vietnamese. In the post–Civil War environment, Anglo-Americans increased the pressure for Anglicization, and in 1921, English was shortly made the sole language of instruction in Louisiana schools before a policy of multilingualism was revived in 1974. Louisiana has never had an official language, and the state constitution enumerates "the right of the people to preserve, foster, and promote their respective historic, linguistic, and cultural origins."

Based on national averages, Louisiana frequently ranks low among U.S. states in terms of health, education, and development, with high rates of poverty and homicide. In 2018, Louisiana was ranked as the least healthy state in the country, with high levels of drug-related deaths. It also has had the highest homicide rate in the United States since at least the 1990s.