Marathon is a city situated in the middle of the Florida Keys within Monroe County, Florida, United States. It occupies a series of keys near the geographic center of the archipelago, extending from the east end of the Seven Mile Bridge to the west end of Tom's Harbor Bridge. The municipality includes numerous islands such as Boot Key, Knight's Key, and Vaca Key, with city limits reaching 1,200 feet into adjacent waters. Its boundaries exclude portions within Key Colony Beach, positioning it as a central hub between Key Largo and Key West.

Although the area has been settled for some time, Marathon is a relatively new municipality, officially incorporated in 1999. The name originates from the Florida East Coast Railroad era, where workers described their unrelenting pace as a marathon. Playwright Witter Bynner is credited with proposing the name for the local station, inspired by Byron's poetry. This historical naming convention stuck, transforming a railroad stop designation into the official identity of the growing community in the Keys.

The city features a major business district near Mile Marker 50 on the Overseas Highway, anchored by the Marathon Publix and the United States Post Office. Notable landmarks include Bonefish Tower, the tallest building in the Keys at 143 feet, and Sombrero Country Club, the region's only country club. The area also contains Crane Point Museum, preserving one of the last untouched tropical hardwood hammocks, and hosts specialized facilities like The Turtle Hospital, which rescues and rehabilitates injured sea turtles.

Marathon serves as a major sport fishing destination, with charter boats departing for both the Gulf of Mexico and the Atlantic Ocean. Its bountiful reefs attract divers, snorkelers, and spearfishers, while the surrounding waters support lobster tickling activities. The local economy relies heavily on the ocean, making seafood a staple at most restaurants. Visitors also come to observe unique wildlife, including the hard-to-find Antillean nighthawk near the airport and gray kingbirds along telephone wires.

As of 2020, Marathon had a population of 9,689 residents living in a tropical savanna climate with no record of snow or frost. The city is served by U.S. Highway 1, known locally as the Overseas Highway, and Florida Keys Marathon Airport. Public transit options include Metrobus Route 301 and Key West Transit. The economy is bolstered by tourism and maritime activities, supported by a branch of the Monroe County Public Library System completed in 2021.