USS Ajax, originally named USS Manayunk after a town in Pennsylvania, was a single-turreted Canonicus-class monitor built for the Union Navy during the American Civil War. Completed after the end of the war, Ajax was laid up until 1871, although she received her new name in 1869. The ship was briefly activated in 1871, before a much longer commission began in 1874–1875. She was assigned to the North Atlantic Squadron during this time. Ajax was again placed in reserve in 1891. The ship was on militia duty when the Spanish–American War began and she was recommissioned in 1898, to defend Baltimore, Maryland, although she was decommissioned later in the year before the necessary refit could be completed. Ajax was sold for scrap in 1899.