The 1935 Labor Day hurricane was an extremely powerful and devastating Atlantic hurricane that struck the southeastern United States in early September 1935. For several decades, it was the most intense Atlantic hurricane on record in terms of barometric pressure, 1-minute sustained winds, and the strongest at landfall by 1-minute sustained winds. The fourth tropical cyclone, third tropical storm, second hurricane, and second major hurricane of the 1935 Atlantic hurricane season, it is one of four Category 5 hurricanes on record to strike the contiguous United States, along with Hurricane Camille (1969), Hurricane Andrew  (1992), and Hurricane Michael (2018).

The hurricane intensified rapidly during its time, passing near Long Key on Labor Day evening, September 2. The region was swept by a massive storm surge as the eye passed over the area. The waters quickly receded after carving new channels connecting the bay with the ocean; however, gale-force winds and rough seas persisted into Tuesday, disrupting rescue efforts. The storm continued northwestward along the Florida west coast, weakening before making its second landfall near Cedar Key, Florida, on September 4.

The hurricane caused catastrophic damage in the upper Florida Keys, as a storm surge of approximately 18 to 20 feet (5.5 to 6.1 m) swept over the low-lying islands. The hurricane's strong winds and the surge destroyed nearly all the structures between Tavernier and Marathon. The town of Islamorada was obliterated. Portions of the Key West Extension of the Florida East Coast Railway were severely damaged or destroyed. In addition, many veterans died in work camps created for the construction of the Overseas Highway, in part due to poor working conditions. The hurricane also caused more damage in northwest Florida, Georgia, and the Carolinas.

An area of disturbed weather developed northeast of the Turks Islands toward the end of August. By August 31, a definite tropical depression appeared near Long Island in the southeastern Bahamas and quickly intensified. It reached hurricane intensity near the south end of Andros Island on September 1. The storm then explosively intensified and turned toward the Florida Keys at a speed of 10 mph. The storm had an eye 9–10 miles (14–16 km) across. The storm made landfall late on September 2 near Long Key, at peak intensity, with an intensity of 892 millibars (26.3 inHg) and 1-minute sustained winds of 185 mph (298 km/h). After leaving the Keys, the storm weakened as it skirted the Florida gulf coast, making a second landfall at Cedar Key. The storm sped up and rapidly weakened over the Mid-Atlantic states, causing heavy rainfall, with the highest total being 16.7 inches (420 mm) in Easton, Maryland. The storm finally emerged over the open Atlantic near Cape Henry. The storm continued into the North Atlantic Ocean, where it merged with an extratropical cyclone on September 10.

The first recorded instance of an aircraft flown for the specific purpose of locating a hurricane occurred on the afternoon of September 2, 1935. The Weather Bureau's 1:30 PM advisory placed the center of the hurricane at north latitude 23° 20', west longitude 80° 15', moving slowly westward. This was about 27 miles (43 km) north of Isabela de Sagua, Villa Clara, Cuba, and 145 miles (233 km) east of Havana. Captain Leonard Povey of the Aviation Corps of the Cuban Army (Cuerpo de Aviación del Ejército de Cuba) volunteered to investigate the threat to the capital. Flying a Curtis Hawk II, Captain Povey, an American expatriate, who was the Aviation Corps' chief training officer, observed the storm north of its reported position. Because he was flying an open-cockpit biplane, he opted not to fly into the storm. He later proposed an aerial hurricane patrol. Nothing further came of this idea until June 1943, when Colonel Joe Duckworth and Lieutenant Ralph O'Hair flew into a hurricane near Galveston, Texas.

The Labor Day hurricane was the most intense tropical cyclone known to make landfall in the Western Hemisphere, having the lowest sea level pressure ever officially recorded on land—a central pressure of 892 millibars (26.3 inHg)—suggesting an intensity of between 162 and 164 knots (186 and 189 mph). The somewhat compensating effects of a slow (7 knots, 8.1 mph) translational velocity along with an extremely tiny radius of maximum wind (5 nmi, 5.8 miles) led to an analyzed intensity at landfall of 160 knots (184 mph; 296 km/h). The 1935 Labor Day hurricane is also tied with 2019's Hurricane Dorian for the highest intensity for a landfalling Atlantic hurricane in HURDAT2, as 1969's Hurricane Camille has been reanalyzed in 2014 to have the third highest landfalling intensity with 150 kn (173 mph; 278 km/h) winds.

Northeast storm warnings were ordered displayed from Fort Pierce to Fort Myers in the September 1, 9:30 AM Weather Bureau advisory. Upon receipt of this advisory the U.S. Coast Guard Station, Miami, FL, sent a plane along the coast to advise boaters and campers of the impending danger by dropping message blocks. A second flight was made on Sunday afternoon. After, planes were placed in the hangar and its door closed at 10:00 AM Monday.

The 3:30 AM advisory, September 2 (Labor Day), predicted that the disturbance "will probably pass through the Florida Straits Monday" and cautioned "against high tides and gales Florida Keys and ships in path". The 1:30 PM advisory ordered hurricane warnings for the Key West district which extended north to Key Largo. At around 2:00 PM, Fred Ghent, Assistant Administrator, Florida Emergency Relief Administration, requested a special train to evacuate the veterans work camps located in the upper keys.