Hurricane Katrina was an extremely powerful and devastating tropical cyclone that killed 1,392 people and caused damage estimated at $125 billion, particularly in and around the city of New Orleans, in late August 2005. It is tied with Hurricane Harvey as being the costliest tropical cyclone in the Atlantic basin. Katrina was the twelfth tropical cyclone, the fifth hurricane, the third major hurricane, and the second Category 5 hurricane of the 2005 Atlantic hurricane season. It was also the fourth-most intense Atlantic hurricane to make landfall in the contiguous United States, as measured by barometric pressure.

Katrina formed on August 23, 2005, with the merger of a tropical wave and the remnants of a tropical depression. After briefly strengthening to a tropical storm over south Florida, Katrina entered the Gulf of Mexico on August 26 and rapidly intensified to a Category 5 hurricane before weakening to a Category 3 at its landfall on August 29 near Buras-Triumph, Louisiana.

Eighty percent of New Orleans, as well as large areas in neighboring parishes, were flooded. It is estimated that about 100,000 to 150,000 people remained in the City of New Orleans, despite mandatory evacuation orders. The flooding prompted a massive national and international response effort, including federal, local, and private rescue operations. The largest loss of life was due to flooding caused by engineering flaws in the federally built hurricane protection system, particularly the levees around New Orleans. Investigators concluded that the U.S. Army Corps of Engineers, tasked by Congress in the Flood Control Act of 1965 to design and build the region's hurricane protection, was responsible for the breached floodwalls. Later, a federal appeals court ruled that the Army Corps, despite being responsible, could not be held financially liable due to the Flood Control Act of 1928.

The emergency response from federal, state, and local governments was widely criticized, leading to the resignation of Federal Emergency Management Agency (FEMA) director Michael D. Brown and New Orleans Police Department (NOPD) superintendent Eddie Compass. Many other government officials faced criticism for their responses, especially New Orleans mayor Ray Nagin, Louisiana governor Kathleen Blanco, and President George W. Bush. However, several agencies, such as the United States Coast Guard (USCG), National Hurricane Center (NHC), and National Weather Service (NWS), were commended for their actions, with the NHC being particularly praised for its accurate forecasts well in advance.

The destruction and loss of life caused by the storm prompted the name Katrina to be retired by the World Meteorological Organization in April 2006. On January 4, 2023, the NHC updated the Katrina fatality data based on a 2014 report, which reduced the total number from an estimated 1,833 to 1,392.

Hurricane Katrina originated from the merger of a tropical wave and the mid-level remnants of Tropical Depression Ten on August 19, 2005, near the Lesser Antilles. On August 23, the disturbance organized into Tropical Depression Twelve over the southeastern Bahamas. The storm strengthened into Tropical Storm Katrina on the morning of August 24. The tropical storm moved towards Florida and became a hurricane only two hours before making landfall between Hallandale Beach and Aventura on the morning of August 25. The storm weakened over land, but it regained hurricane status about one hour after entering the Gulf of Mexico, and it continued strengthening over open waters. On August 27, the storm reached Category 3 intensity on the Saffir–Simpson hurricane wind scale, becoming the third major hurricane of the season. An eyewall replacement cycle disrupted the intensification but caused the storm to nearly double in size. Thereafter, Katrina rapidly intensified over the "unusually warm" waters of the Loop Current, from a Category 3 hurricane to a Category 5 hurricane in just nine hours.

After attaining Category 5 hurricane status on the morning of August 28, Katrina reached its peak strength at 1800 UTC, with maximum sustained winds of 175 mph (280 km/h) and a minimum central pressure of 902 mbar (26.64 inHg). The pressure measurement made Katrina the fifth most intense Atlantic hurricane on record at the time, only to be surpassed by Hurricanes Rita and Wilma later in the season; it was also the strongest hurricane ever recorded in the Gulf of Mexico at the time, before Rita broke the record. The hurricane subsequently weakened due to another eyewall replacement cycle, and Katrina made its second landfall at 1110 UTC on August 29, as a high-end Category 3 hurricane with sustained winds of 125 mph (201 km/h), near Buras-Triumph, Louisiana. At landfall, hurricane-force winds extended outward 120 miles (190 km) from the center and the storm's central pressure was 920 mbar (27 inHg). After moving over southeastern Louisiana and then Breton Sound, it made its third and final landfall near the Louisiana–Mississippi border, with Category 3 sustained winds of 120 mph (195 km/h). Katrina maintained hurricane strength well into Mississippi, finally weakening to tropical storm strength more than 150 miles (240 km) inland near Meridian, Mississippi. It was downgraded to a tropical depression near Clarksville, Tennessee; its remnants were absorbed by a cold front in the eastern Great Lakes region on August 31. The resulting extratropical storm moved rapidly to the northeast and affected eastern Canada.

The United States Coast Guard began pre-positioning resources in a ring around the expected impact zone and activated more than 400 reservists. On August 27, it moved its personnel out of the New Orleans region prior to the mandatory evacuation. Aircrews from the Aviation Training Center, in Mobile, staged rescue aircraft from Texas to Florida. All aircraft were returning towards the Gulf of Mexico by the afternoon of August 29. Aircrews, many of whom lost their homes during the hurricane, began a round-the-clock rescue effort in New Orleans, and along the Mississippi and Alabama coastlines.