Key Largo is an unincorporated area and census-designated place situated in Monroe County, Florida, within the United States. It occupies the island of Key Largo in the upper Florida Keys, sharing this landmass with Tavernier to the southwest and North Key Largo to the northeast. As a significant community in the region, it serves as the first island and town reached from the Overseas Highway when traveling toward Key West.

The name Key Largo derives from the Spanish Cayo Largo, meaning long key. It holds historical significance as the location of one of the stations for the Overseas Railroad. The area is also notable for establishing the first undersea park in the United States in 1963, known as the John Pennekamp Coral Reef State Park, which was designed to protect marine life and extensive coral reefs.

The community is defined by its coastal geography and tourism infrastructure. U.S. Route 1 runs the length of the area, connecting it to Miami and Key West. Key landmarks include the Dagny Johnson Key Largo Hammock Botanical State Park and the African Queen vessel, which offers canal cruises. The John Pennekamp Coral Reef State Park remains a central feature, offering snorkeling, scuba diving, and camping facilities.

Key Largo is renowned as a diving mecca, attracting visitors for water-related activities such as snorkeling, glass bottom boat tours, parasailing, and fishing. The area also features motorcycle rentals and the Florida Keys Overseas Heritage Trail. Its cultural appeal is enhanced by its association with the 1948 film starring Humphrey Bogart and the 1981 hit song by Bertie Higgins, drawing tourists interested in its maritime heritage.

The population was 12,447 at the 2020 census, reflecting growth from previous decades. The local economy includes tourism, with many dive companies operating from the area. Access is primarily via U.S. Route 1 or by flying into Miami or Fort Lauderdale. Key Largo experiences a tropical monsoon climate according to the Köppen classification, supporting its year-round appeal for outdoor and marine enthusiasts.