Hurricane Katrina devastates much of the U.S. Gulf Coast from Louisiana to the Florida Panhandle, killing an estimated 1,836 people and causing over $108 billion in damage.

As the center of Hurricane Katrina passed southeast of New Orleans on August 29, 2005, winds downtown were in the Category 1 range with frequent intense gusts. The storm surge caused approximately 23 breaches in the drainage canal and navigational canal levees and flood walls. As mandated in the Flood Control Act of 1965, responsibility for the design and construction of the city's levees belongs to the United States Army Corps of Engineers and responsibility for their maintenance belongs to the Orleans Levee Board. The failures of levees and flood walls during Katrina are considered by experts to be the worst engineering disaster in the history of the United States. By August 31, 2005, 80% of New Orleans was flooded, with some parts under 15 feet (4.6 m) of water. The famous French Quarter and Garden District escaped flooding because those areas are above sea level. The major breaches included the 17th Street Canal levee, the Industrial Canal levee, and the London Avenue Canal flood wall. These breaches caused the majority of the flooding, according to a June 2007 report by the American Society of Civil Engineers. The flood disaster halted oil production and refining which increased oil prices worldwide.

Between 80 and 90 percent of the residents of New Orleans were evacuated before the hurricane struck, testifying to some of the success of the evacuation measures. Despite this, many remained in the city, mainly those who did not have access to personal vehicles or who were isolated from the dissemination of news from the local governments. The Louisiana Superdome was used to house and support some of those who were unable to evacuate. Television shots frequently focused on the Superdome as a symbol of the flooding occurring in New Orleans.

The disaster had major implications for a large segment of the population, economy, and politics of the entire United States. It has prompted a Congressional review of the Army Corps of Engineers and the failure of portions of the federally built flood protection system which experts agree should have protected the city's inhabitants from Katrina's surge. Katrina has also stimulated significant research in the academic community into urban planning, real estate finance, and economic issues in the wake of a natural disaster.

Hurricane Katrina was a large and destructive Category 5 Atlantic hurricane that caused over 1,800 fatalities and $125 billion in damage in late August 2005, especially in the city of New Orleans and the surrounding areas. It was at the time the costliest tropical cyclone on record and is now tied with 2017's Hurricane Harvey. The storm was the twelfth tropical cyclone, the fifth hurricane, and the third major hurricane of the 2005 Atlantic hurricane season, as well as the fourth-most intense Atlantic hurricane on record to make landfall in the contiguous United States.

Katrina originated on August 23, 2005, as a tropical depression from the merger of a tropical wave and the remnants of Tropical Depression Ten. Early the following day, the depression intensified into a tropical storm as it headed generally westward toward Florida, strengthening into a hurricane two hours before making landfall at Hallandale Beach on August 25. After briefly weakening to tropical storm strength over southern Florida, Katrina emerged into the Gulf of Mexico on August 26 and began to rapidly intensify. The storm strengthened into a Category 5 hurricane over the warm waters of the Gulf of Mexico before weakening to Category 3 strength at its second landfall on August 29 over southeast Louisiana and Mississippi.

Flooding, caused largely as a result of fatal engineering flaws in the flood protection system (levees) around the city of New Orleans, precipitated most of the loss of lives. Eventually, 80% of the city, as well as large tracts of neighboring parishes, were inundated for weeks. The flooding also destroyed most of New Orleans's transportation and communication facilities, leaving tens of thousands of people who had not evacuated the city prior to landfall stranded with little access to food, shelter, or other basic necessities. The scale of the disaster in New Orleans provoked massive national and international response efforts; federal, local, and private rescue operations evacuated displaced persons out of the city over the following weeks. Multiple investigations in the aftermath of the storm concluded that the U.S. Army Corps of Engineers, which had designed and built the region's levees decades earlier, was responsible for the failure of the flood-control systems, though federal courts later ruled that the Corps could not be held financially liable because of sovereign immunity in the Flood Control Act of 1928.The emergency response from federal, state, and local governments was widely criticized, resulting in the resignations of Federal Emergency Management Agency (FEMA) director Michael D. Brown and New Orleans Police Department (NOPD) Superintendent Eddie Compass. Many other government officials were criticized for their responses, especially New Orleans Mayor Ray Nagin, Louisiana Governor Kathleen Blanco, and President George W. Bush, while several agencies, including the United States Coast Guard (USCG), National Hurricane Center (NHC), and National Weather Service (NWS), were commended for their actions. The NHC was especially applauded for providing accurate forecasts well in advance. Katrina was the earliest 11th named storm on record before being surpassed by Tropical Storm Kyle on August 14, 2020.