USS Constitution, also known as Old Ironsides, is a three-masted wooden-hulled heavy frigate of the United States Navy. She is the world's oldest commissioned warship still afloat. She was launched in 1797, one of six original frigates authorized for construction by the Naval Act of 1794 and the third constructed. The name "Constitution" was among ten names submitted to President George Washington by Secretary of War Timothy Pickering in March or May the frigates that were to be constructed. Joshua Humphreys designed the frigates to be the young Navy's capital ships, and so Constitution and her sister ships were larger and more heavily armed and built than standard frigates of the period. She was built at Edmund Hartt's shipyard in the North End of Boston, Massachusetts. Her first duties were to provide protection for American merchant shipping during the Quasi-War with France and to defeat the Barbary pirates in the First Barbary War.

Constitution is most noted for her actions during the War of 1812 with the United Kingdom, when she captured numerous British merchantmen and five warships: HMS Guerriere, Java, Pictou, Cyane, and Levant. The capture of Guerriere earned her the nickname "Old Ironsides", adding on the public adoration that had repeatedly saved her from scrapping. She continued to serve as flagship in the Mediterranean and African squadrons, and she circled the world in the 1840s. During the American Civil War, she served as a training ship for the United States Naval Academy. She carried American artwork and industrial displays to the Paris Exposition of 1878.

Constitution was retired from active service in 1881 and served as a receiving ship until being designated a museum ship in 1907. In 1934, she completed a three-year, 90-port tour of the nation. She sailed under her own power for her 200th birthday in 1997, and again in August 2012 to commemorate the 200th anniversary of her victory over Guerriere.

Constitution's stated mission today is to promote understanding of the Navy's role in war and peace through educational outreach, historical demonstration, and active participation in public events as part of the Naval History and Heritage Command. As she is a fully commissioned Navy ship, her crew of 75 officers and sailors participate in ceremonies, educational programs, and special events while keeping her open to visitors year round and providing free tours. The officers and crew are all active-duty Navy personnel, and the assignment is considered to be special duty. She is usually berthed at Pier 1 of the former Charlestown Navy Yard at one end of Boston's Freedom Trail.

In 1785, Barbary pirates, most notably from Algiers, began to seize American merchant vessels in the Mediterranean Sea. In 1793 alone, 11 American ships were captured and their crews and stores held for ransom. To combat this problem, proposals were made for warships to protect American shipping, resulting in the Naval Act of 1794. The act provided funds to construct six frigates, but it included a clause that the construction of the ships would be halted if peace terms were agreed to with Algiers.

Joshua Humphreys' design was unusual for the time, being deep, long on keel, narrow of beam (width), and mounting very heavy guns. The design called for diagonal riders intended to restrict hogging and sagging while giving the ships extremely heavy planking. This design gave the hull a greater strength than a more lightly built frigate. It was based on Humphrey's realization that the fledgling United States could not match the European states in the size of their navies, so they were designed to overpower any other frigate while escaping from a ship of the line.

Her keel was laid down on 1 November 1794 at Edmund Hartt's shipyard in Boston, Massachusetts, under the supervision of Captain Samuel Nicholson, master shipwright Colonel George Claghorn and Foreman Prince Athearn of the Martha's Vineyard Athearns. Constitution's hull was built 21 inches (530 mm) thick and her length between perpendiculars was 175 ft (53 m), with a 204 ft (62 m) length overall and a width of 43 ft 6 in (13.26 m). In total, 60 acres (24 ha) of trees were needed for her construction. Primary materials consisted of pine and oak, including southern live oak which was cut from Gascoigne Bluff and milled near St. Simons Island, Georgia. Enslaved workers were used to harvest the oak used for the ship's construction, and USS Constitution Museum historian Carl Herzog stated that "the forced labor of enslaved people was an expediency that Navy officials and contractors saw as fundamental to the job... enslaved people were essential to the construction of naval warships built to secure the very American freedoms they were denied."

A peace accord was announced between the United States and Algiers in March 1796, and construction was halted in accordance with the Naval Act of 1794. After some debate and prompting by President Washington, Congress agreed to continue funding the construction of the three ships nearest to completion: United States, Constellation, and Constitution. Constitution's launching ceremony on 20 September 1797 was attended by President John Adams and Massachusetts Governor Increase Sumner. Upon launch, she slid down the ways only 27 feet (8.2 m) before stopping; her weight had caused the ways to settle into the ground, preventing further movement. An attempt two days later resulted in only 31 feet (9.4 m) of additional travel before the ship again stopped. After a month of rebuilding the ways, Constitution finally slipped into Boston Harbor on 21 October 1797, with Captain James Sever breaking a bottle of Madeira wine on her bowsprit.