Fort Carroll is a 3.4 acre (14,000 m²) artificial island and abandoned fort in the middle of the Patapsco River, just south of Baltimore, Maryland.

The structure was designed by then Brevet-Colonel Robert E. Lee and named for Charles Carroll of Carrollton, a signer of the Declaration of Independence. Construction of the fort began in 1848, under the supervision of Brevet-Colonel Lee, U.S. Army Corps of Engineers. Fort Carroll was important for the defense of Baltimore -- before the fort was created, the only military defensive structure between Baltimore and the Chesapeake Bay was Fort McHenry.

The government abandoned the fort as a military post in 1920, and the island was declared excess property in 1923. However, the War Department took no immediate steps to sell the land. In May 1958, a Baltimore attorney purchased the island for $10,000, but development plans never materialized. The fort now is deserted.