barrier island, Lee county, southwestern Florida, U.S., about 20 miles (30 km) southwest of Fort Myers. It lies between the Intracoastal Waterway (north) and the Gulf of Mexico (Mexico, Gulf of) (south) and is connected to the mainland by a causeway. The island is about 12 miles (20 km) long and 3 miles (5 km) wide. A large part of its northern side is occupied by J.N. "Ding" Darling National Wildlife Refuge.

Sanibel Island is known for its relatively pristine environmental condition that features mangrove swamps and abundant wildlife, including alligators, sea turtles, and dozens of varieties of birds. Through community efforts, the island has avoided much of the large-scale development that has occurred throughout the rest of Florida's coastal areas. Development began in earnest after the causeway opened in 1963, but in 1974 the city of Sanibel incorporated the surrounding area and adopted strict zoning laws and building regulations that restricted the number of buildings, their height, and how close to the beach they could be built. The city also has prohibited billboards, chain restaurants, and lights that can be seen from the beach and has imposed guidelines to protect native plants and animals from human interference. The island is a popular tourist destination renowned for the seashells that can be gathered on its beaches; the Bailey-Matthews Shell Museum is dedicated to the subject. Nearby Captiva Island, off Sanibel's northwestern end, is also a resort area. Pop. (1990) 5,468; (2000) 6,064.