Gustavus Vasa Fox (13 June 1821 – 29 October 1883) was a American navy officer.

Born at Saugus, Massachusetts, Fox was appointed midshipman 12 January 1838. During the Mexican-American War, he served in the brig Washington in the squadron of Commodore Matthew Perry and took active part in the second expedition against Tobasco, Mexico , 14-16 January 1847, which resulted in the capture of that town. He was in command of several mail steamers and after his resignation 30 July 1856, engaged in the manufacture of woolen materials.

At the start of the American Civil War he volunteered for service. He was given a temporary appointment in the Navy and was sent in the steamer Baltic to the relief of Major Robert Anderson and the remnant of his command in Fort Sumter, and brought them away.

On 1 August 1861, President Abraham Lincoln appointed him Assistant Secretary of the Navy, an office which he held until the close of the Civil War. In 1866, he was sent on a special mission to Russia and conveyed the congratulations of the President of the United States to Tzar Alexander II upon his escape from assassination. His voyage was made in the monitor Miantonomah which was the first vessel of this class to cross the Atlantic. They were accompanied by Augusta

In 1882 he made a publication suggesting Samana Cay in the Bahamas to be Guanahani, or San Salvador, the first island Christopher Columbus reached at his discovery of the Americas. Little attention was paid to it until 1986, when the National Geographic Society also appointed Samana Cay to be San Salvador.

He died at Lowell, Massachusetts.

Two ships have been named USS Fox for him.