San Marco Island is a city in Collier County, Florida, United States, located on an island by the same name in the Gulf of Mexico off the coast of Southwest Florida. It is a principal city of the Naples–Marco Island Metropolitan Statistical Area.
San Marco Island is the largest barrier island within Southwest Florida's Ten Thousand Islands area extending southerly to Cape Sable. Parts of the island have some scenic, high elevations relative to the generally flat south Florida landscape. Like the city of Naples to the north, San Marco Island has a tropical climate; specifically a tropical wet and dry or savanna type (Aw under the Köppen system). It is known for distinct wet and dry seasons, with most of the rainfall falling between the months of June and October.