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is a city on Knight's Key, Boot Key, Key Vaca, Fat Deer Key, Long Point Key, Crawl Key and Grassy Key islands in the middle Florida Keys, in Monroe County, Florida, in the United States. As of the 2000 census, the city had a total population of 10,255. As of 2005, the population recorded by the U.S. Census Bureau is 10,626.
According to the United States Census Bureau, the city has a total area of 25.0 km² (9.6 mi²). 22.4 km² (8.6 mi²) of it is land and 2.6 km² (1.0 mi²) of it (10.37%) is water. Its city limits extend 1200' from land into the adjacent waters.
Boot Key Harbor is a natural body of water between Boot Key and Key Vaca, entirely within the Marathon city limits.