Lafayette (/ˌlæfiˈjɛt, ˌlɑːf-/ LA(H)-fee-YET, French: [lafajɛt]) is the most populous city in and the parish seat of Lafayette Parish in the U.S. state of Louisiana, located along the Vermilion River. It is Louisiana's fourth-most populous city with a 2020 census population of 121,374; the consolidated city-parish's population was 241,753 in 2020. The Lafayette metropolitan area was Louisiana's third largest metropolitan statistical area with a population of 478,384 at the 2020 census. The Acadiana region containing Lafayette is the largest population and economic corridor between Houston, Texas and New Orleans.

Originally established as Vermilionville in the 1820s and incorporated in 1836, Lafayette developed as an agricultural community until the introduction of retail and entertainment centers, and the discovery of oil in the area in the 1940s. Since the discovery of oil, the city and parish have had the highest number of workers in the oil and natural gas industry in Louisiana as of 2018. With the issuance of a bond ordinance for a series of roads connecting nearby settlements, the establishment of the University of Louisiana System's Lafayette campus, and the continued diversification of its economy, Lafayette and its metropolitan area have experienced population growth since the 1840 census and was promptly nicknamed "The Hub City." The city and parish of Lafayette are also known as the "Heart of Acadiana."

As a result of its growth, the city and region have become major centers for the technology industry; Lafayette also became a major center for health care and social services, aerospace, banking, and retail. Notable corporations with headquarters or a large presence in the Lafayette area have included Amazon, Brookshire Grocery Company, CGI, JP Morgan Chase, Ochsner Health System, Petroleum Helicopters International, and Rouses Markets.

Lafayette is home to a diverse population from Louisiana Creole and Cajun backgrounds. The city and region's cultural icons include Alexandre Mouton House, Brandt House, Charles H. Mouton House, the Cathedral of Saint John the Evangelist, Daigle House, First United Methodist Church, Holy Rosary Institute, Hope Lodge No. 145, and Old Lafayette City Hall. Its educational institutions include the University of Louisiana at Lafayette, South Louisiana Community College, and Remington College.

Lafayette is named after Marquis de Lafayette. Little is known about early settlements or if the area had a different name prior to European colonization. The city was originally founded in 1821 as Vermilionville.

The Attakapa Native Americans inhabited this area at the time of the first European encounter. French colonists founded the first European settlement, Petit Manchac, a trading post along the Vermilion River. In the mid-to-late eighteenth century, numerous Acadian refugees settled in this area, after being expelled from Canada after Great Britain defeated France in the Seven Years' War. They intermarried with other settlers, forming what became known as Cajun culture, which maintained use of the French language and adherence to the Roman Catholic Church.

Jean Mouton, an Acadian settler, donated land to the Roman Church for construction of a small Catholic chapel at this site. In 1824, this area was selected for the Lafayette Parish seat and was named Vermilionville, for its location on the river; in 1836, the Louisiana State Legislature approved its incorporation.

The area was initially developed by Europeans for agriculture, primarily sugar plantations, which depended on the labor of numerous enslaved Africans and African Americans. They made up a large percentage of the antebellum population. According to U.S. census data in 1830, some 41 percent of the population of Lafayette Parish was enslaved. By 1860, the enslaved population had increased to 49.6 percent. Some free people of color lived in Lafayette Parish, as well; they made up 3 percent to a low of 2.4 percent between 1830 and 1860.