USS Monitor was an ironclad warship built for the Union 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 (built on the hull of the scuttled steam frigate USS Merrimack) 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.

The remainder of the ship was designed by Swedish-born engineer and inventor John Ericsson, and built in only 101 days in Brooklyn, New York on the East River beginning in late 1861. Monitor presented a new concept in ship design and employed a variety of new inventions and innovations in ship building that caught the attention of the world. The impetus to build Monitor was prompted by the news that the Confederates were building an iron-plated armored vessel named the Virginia in the old Federal naval shipyard at Gosport, near Norfolk, that could effectively engage the Union ships blockading Hampton Roads harbor and the James River leading northwest to Richmond (capital of the Confederacy). They could ultimately advance unchallenged on Washington, D.C., up the Potomac River and other seacoast cities. Before Monitor could reach Hampton Roads, the Confederate ironclad had already destroyed the sail frigates USS Cumberland and USS Congress and had run the steam frigate USS Minnesota aground. That night, Monitor arrived and, just as Virginia set to finish off Minnesota and St. Lawrence on the second day, the new Union ironclad confronted the Confederate ship, preventing her from wreaking further destruction on the wooden Union ships. A four-hour battle ensued, each ship pounding the other with close-range cannon fire, although neither ship could destroy or seriously damage the other. This was the first battle fought between armored warships and marked a turning point in naval warfare.

The Confederates were forced to scuttle and destroy Virginia as they withdrew in early May 1862 from Norfolk and its naval shipyard, while Monitor sailed up the James River to support the Union Army during the Peninsula Campaign under General-in-Chief George B. McClellan. The ship participated in the Battle of Drewry's Bluff later that month, and remained in the area giving support to General McClellan's forces on land until she was ordered to join the Union Navy blockaders off North Carolina in December. On her way there, she foundered while under tow during a storm off Cape Hatteras on the last day of the year. Monitor's wreck was discovered in 1973 and has been partially salvaged. Her guns, gun turret, engine, and other relics are on display at the Mariners' Museum in Newport News, Virginia, a few miles from the site of her most important military action.

An ironclad is a steam-propelled warship protected by iron or steel armor plates, constructed from 1859 to the early 1890s. The ironclad was developed as a result of the vulnerability of wooden warships to explosive or incendiary shells. The first ironclad battleship, Gloire, was launched by the French Navy in November 1859 - narrowly pre-empting the British Royal Navy. After the first clashes of ironclads (both with wooden ships and with one another) took place in 1862 during the American Civil War, it became clear that the ironclad had replaced the unarmored ship of the line as the most powerful warship afloat. This type of ship came to be very successful in the American Civil War.Ironclads were designed for several roles, including as high seas battleships, long-range cruisers, and coastal defense ships. Rapid development of warship design in the late 19th century transformed the ironclad from a wooden-hulled vessel that carried sails to supplement its steam engines into the steel-built, turreted battleships and cruisers familiar in the 20th century. This change was pushed forward by the development of heavier naval guns, more sophisticated steam engines, and advances in metallurgy which made steel shipbuilding possible.

The quick pace of change meant that many ships were obsolete almost as soon as they were finished, and that naval tactics were in a state of flux. Many ironclads were built to make use of the ram, the torpedo, or sometimes both (as in the case with smaller ships and later torpedo boats), which a number of naval designers considered the important weapons of naval combat. There is no clear end to the ironclad period, but towards the end of the 1890s the term ironclad dropped out of use. New ships were increasingly constructed to a standard pattern and designated battleships or armored cruisers.