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.

They were first used in warfare in 1862 during the American Civil War, when ironclads operated against wooden ships and, in a historic confrontation, against each other at the Battle of Hampton Roads in Virginia. Their performance demonstrated that the ironclad had replaced the unarmored ship of the line as the most powerful warship afloat. Ironclad gunboats became very successful in the American Civil War.

Ironclads were designed for several uses, 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 that 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 toward the end of the 1890s, the term ironclad dropped out of use. New ships were increasingly constructed to a standard pattern and designated as battleships or armored cruisers.