Located on the Florida Reef Tract, 3.5 miles due south of Boot Key.

Both Gauld (1774) and Blunt (1846) charts show Cayo Sombrero. J.W. Norie, in his "Piloting Directions for the Gulf of Florida, the Bahama Bank, & Islands" (1828) states: "About 5 miles S. 1/4 E. from the west end of Cayo Vacas, there is a small sandy kay on the reef, called by the Spaniards Cayo Sombrero; this is the easternmost kay on the reef."

Site of the Sombrero Key Lighthouse, completed in 1858.