The Passaic-class ironclad monitors of the U.S. Navy saw service in the U.S. Civil War and the Spanish–American War. The class was an improved version of USS Monitor equipped with a 15-inch Dahlgren gun in place of one of the 11-inch guns.

USS Monitor was an ironclad warship built for the United States Navy during the American Civil War and completed in early 1862, the first such ship commissioned by the Navy. Monitor played a central role in the Battle of Hampton Roads on 9 March under the command of Lieutenant John L. Worden, where she fought the casemate ironclad CSS Virginia to a stalemate. The design of the ship was distinguished by its revolving turret, which was designed by American inventor Theodore Timby; it was quickly duplicated and established the monitor class and type of armored warship built for the American Navy over the next several decades.