Key Largo is an island in the upper Florida Keys archipelago and, at long, the largest of the Keys. It is also the northernmost of the Florida Keys in Monroe County, and the northernmost of the Keys connected by U.S. Highway 1 (the Overseas Highway). Its earlier Spanish name was Cayo Largo, meaning Long Key.

Key Largo is connected to the mainland in Miami-Dade County by two routes. The Overseas Highway, which is U.S. Highway 1, enters Key Largo at Jewfish Creek near the middle of the island and turns southwest. Card Sound Road connects to the northern part of Key Largo at Card Sound Bridge and runs southeastward to connect with County Road 905, which runs southwest and joins U.S. 1 at about mile marker 106. These routes originate at Florida City on the mainland.

Key Largo is a popular tourist destination and calls itself the "Diving Capital of the World" because the living coral reef a few miles offshore attracts thousands of scuba divers and sport-fishing enthusiasts.

Key Largo's proximity to the Everglades also makes it a premier destination for kayakers and ecotourists. Automotive and highway pioneer and Miami Beach developer Carl G. Fisher built Key Largo's famous Caribbean Club in 1938 as his last project.

The island gained fame as the setting for the 1948 Humphrey Bogart-Lauren Bacall film Key Largo, although it was filmed entirely on a Warner Brothers sound stage in Hollywood. The island's post office in Rock Harbor, which had been named after a nearby cove, changed its name to Key Largo after the film's success and pressure from local businesmen. It did this so that everyone living north of Tavernier would have a Key Largo address and the cancellation would say Key Largo instead of Rock Harbor.

There are three census-designated places on the island of Key Largo: North Key Largo, near the Card Sound Bridge, Key Largo, eight or nine miles from the southern end of the island, and Tavernier, at the southern end of the island. Ocean Reef Club is a private gated community and club at the northern end of the island. None of Key Largo is an incorporated municipality, so it is governed at the local level by Monroe County.

Key Largo is situated between Everglades National Park to the north-west and John Pennekamp Coral Reef State Park to the east, the first underwater park in the United States and the site of the only living coral barrier reef in the continental United States.

Geology and geography

The island of Key Largo is an exposed, fossilized remnant of a coral reef that was formed during a period of higher sea level and then uncovered and eroded during a subsequent ice age. The highest elevation is a slight ridge forming the spine of the island, which rises to as high as about 15 feet. The land slopes from the spine down to sea level on the oceanside and bayside.