Saint Louis Cathedral, also known as the Basilica of St. Louis, King of France, has the distinction of being the oldest continuously operating cathedral in the United States. The first church on the site was built in 1718. The third church, built in 1789, was raised to cathedral rank in 1793. It is in the French Quarter of New Orleans, Louisiana, USA, on the Place John Paul II, a promenaded section of Chartres Street that stretches one block between St. Peter Street on the upriver boundary and St. Ann Street on the downriver boundary.

While not the largest or grandest of the city's Roman Catholic churches, this historic Cathedral is an important religious and social center, as well as the seat of the Roman Catholic Archdiocese of New Orleans. On the site of three previous churches, the cathedral was expanded and largely rebuilt in 1850, so little of the 1789 structure remained. Located next to Jackson Square and facing the Mississippi River, the St. Louis Cathedral is at the heart of New Orleans and is one of its most recognizable landmarks. It is often used as the backdrop for newscasts and political speeches featuring the city.

It is situated between the historic buildings of the Cabildo and the Presbytère. One of the few Roman Catholic churches in the United States that fronts a major public square, the location of St. Louis Cathedral demonstrates the French Catholic origin of New Orleans.

Three Roman Catholic churches have been on this spot since 1718. The first church was a crude wooden structure in the early days of the colony. Construction of a larger brick and timber church began in 1725 and was completed in 1727. This church was destroyed, along with a large number of other buildings of the city, in the Great New Orleans Fire (1788) on Good Friday on 21 March 1788. The cornerstone of a new church was laid in 1789 and the building was completed in 1794. In 1793 Saint Louis Church was elevated to cathedral rank, making it one of the oldest cathedrals in the United States. In 1819 a central tower with the clock and bell was added.

Enlarging the building to fit the needs of the growing congregation had been pondered since 1834, and J. N. B. de Pouilly was consulted to design plans for a new building. De Pouilly also designed St. Augustine Church in Tremé, the first church building dedicated as a parish church outside the French Quarter. [The Mortuary Chapel on North Rampart had been dedicated in 1827 as a chapel, and St. Vincent de Paul was established in a little frame church in 1838 but not dedicated.] On March 12, 1849, a contract was made with John Patrick Kirwan to enlarge and restore the cathedral, using De Pouilly's plans.

These specified that everything except the lateral walls and the lower portions of the existing towers on the front facade be demolished. During the reconstruction, it was determined that the sidewalls would have to be demolished also. Then, during construction in 1850, the central tower collapsed. De Pouilly and Kirwan were replaced.As a consequence of these events, very little of the Spanish Colonial structure survived, and the present structure primarily dates to 1850. The bell from the 1819 tower was reused in the new building and remains there today. During the renovation, St. Patrick's Church served as the pro-cathedral for the city.

On 25 April, 1909 a dynamite bomb was set off in the Cathedral, blowing out windows and damaging galleries, but doing less severe damage than might have been expected. The Cathedral suffered damage in the New Orleans Hurricane of 1915. The following year a portion of the foundation collapsed, necessitating the building's closure while repairs were made, from Easter 1916 to Easter 1917.

The cathedral was designated as a minor basilica by Pope Paul VI in 1964. Pope John Paul II visited the cathedral in September 1987.

While Hurricane Katrina did not affect the French Quarter as profoundly as other parts of New Orleans, the high winds managed to displace two large oak trees in St. Anthony's Garden behind the Cathedral. The trees dislodged thirty feet of ornamental gate, while the marble statue of Jesus Christ lost only a forefinger and a thumb. Locals have already declared that the statue of Jesus sacrificed two fingers while flicking the storm away from the city and saving it from total destruction. (Hurricane Katrina was downgraded from a Category 5 to a Category 3 before landfall, when it made a turn to the north just before striking the Louisiana coast.)

The Cathedral experienced its most profound loss due to a hole in the roof from Hurricane Katrina's winds. Water entering the building poured into the Holtkamp pipe organ. The organ was severely damaged. Shortly after the storm, the organ was sent back to Holtkamp to be rebuilt. An electronic substitute was used until June 2008, when the organ was reinstalled in the Cathedral. Originally installed during the Cathedral's extensive renovation in 2004, the organ was donated by longtime choir master and organist Dr. Elise Cambon.