Fort Johnson, Colonial fort on James Island, United States.

Fort Johnson is a state-owned historic site on the northeast point of James Island, Charleston County, South Carolina. Its surviving brick magazine, built in 1765, measures 27 feet long and 20 feet wide. The fort was named after Sir Nathaniel Johnson, Governor of Carolina from 1703 to 1709.

Fort Johnson was built in 1708 by Huguenot refugees to protect the English colony of South Carolina from Spanish and French attacks. Storm damage led to a complete rebuild in 1759, and Patriot forces seized the fort in September 1775. Rebuilt again in the 1790s, it was wrecked by storms during the War of 1812 and later fortified and held by Confederate forces throughout the Civil War.

The fort's name honours Sir Nathaniel Johnson, Governor of Carolina from 1703 to 1709. In September 1775 Patriot forces seized the site, and it was there that the South Carolina state flag was raised for the first time. The brick magazine of 1765, 27 feet by 20 feet, still stands for visitors to see.

Fort Johnson sits on the northeast point of James Island in Charleston County, on the bank of the Ashley River. The brick magazine of 1765, about 27 feet by 20 feet, is the structure to look for on the grounds. The site is state-owned and listed on the National Register of Historic Places.

During the Civil War, Confederate soldiers deliberately buried the fortification to protect it from destruction. The site remained hidden underground until it was rediscovered and excavated in 1931.