Pentagon Barracks, Military barracks in State Capitol grounds, Baton Rouge, United States.

Pentagon Barracks is a complex of four two-story brick buildings arranged in a five-sided shape on the Mississippi River bank in downtown Baton Rouge. The structures now serve as private residences for state lawmakers and house offices of the Louisiana lieutenant governor within the State Capitol grounds.

The site held a British earthwork, Fort New Richmond, which Spanish governor Bernardo de Gálvez captured in 1779 and renamed Fort San Carlos. In 1810 rebels of the Republic of West Florida raised their Bonnie Blue Flag over that fort. Captain James Gadsden designed the brick barracks, built 1819–1825, and the Army demolished Fort San Carlos in 1819.

The nickname comes from the four brick buildings that form four sides of a regular pentagon, with the commissary-warehouse as the fifth side. From 1886 the grounds housed Louisiana State University students as dormitories until the university moved in 1926. Sigma Iota fraternity was founded here on February 27, 1903 as La Estrella de Oro, a secret society for Spanish-American students.

The buildings are part of the State Capitol Complex and can be viewed from the outside, though they are not open to the public as they contain private residences. Visitors should appreciate the architecture and riverfront setting from the public areas of the Capitol Grounds.

The fifth side of the pentagon was a commissary-warehouse built in 1821, torn down within months for faulty construction. The four brick buildings that remain could house one thousand troops. Union soldiers later folded an old Indian mound into earthworks defending the complex, renamed Fort Williams after General Thomas Williams, killed in the 1862 battle.