The Second Amendment (Amendment II) to the United States Constitution protects the right to keep and bear arms. It was ratified on December 15, 1791, along with nine other articles of the Bill of Rights. In District of Columbia v. Heller (2008), the Supreme Court affirmed for the first time that the right belongs to individuals, for self-defense in the home, while also including, as dicta, that the right is not unlimited and does not preclude the existence of certain long-standing prohibitions such as those forbidding "the possession of firearms by felons and the mentally ill" or restrictions on "the carrying of dangerous and unusual weapons". In McDonald v. City of Chicago (2010) the Supreme Court ruled that state and local governments are limited to the same extent as the federal government from infringing upon this right. New York State Rifle & Pistol Association, Inc. v. Bruen (2022) assured the right to carry weapons in public spaces with reasonable exceptions.

The Second Amendment was based partially on the right to keep and bear arms in English common law and was influenced by the English Bill of Rights of 1689. Sir William Blackstone described this right as an auxiliary right, supporting the natural rights of self-defense and resistance to oppression, and the civic duty to act in concert in defense of the state. While both James Monroe and John Adams supported the Constitution being ratified, its most influential framer was James Madison. In Federalist No. 46, Madison wrote how a federal army could be kept in check by state militias, "a standing army ... would be opposed [by] a militia." He argued that state militias "would be able to repel the danger" of a federal army, "It may well be doubted, whether a militia thus circumstanced could ever be conquered by such a proportion of regular troops." He contrasted the federal government of the United States to the European kingdoms, which he described as "afraid to trust the people with arms", and assured that "the existence of subordinate governments ... forms a barrier against the enterprises of ambition".

By January 1788, Delaware, Pennsylvania, New Jersey, Georgia and Connecticut ratified the Constitution without insisting upon amendments. Several amendments were proposed, but were not adopted at the time the Constitution was ratified. For example, the Pennsylvania convention debated fifteen amendments, one of which concerned the right of the people to be armed, another with the militia. The Massachusetts convention also ratified the Constitution with an attached list of proposed amendments. In the end, the ratification convention was so evenly divided between those for and against the Constitution that the federalists agreed to the Bill of Rights to assure ratification. In United States v. Cruikshank (1876), the Supreme Court ruled that, "The right to bear arms is not granted by the Constitution; neither is it in any manner dependent upon that instrument for its existence. The Second Amendments [sic] means no more than that it shall not be infringed by Congress, and has no other effect than to restrict the powers of the National Government." In United States v. Miller (1939), the Supreme Court ruled that the Second Amendment did not protect weapon types not having a "reasonable relationship to the preservation or efficiency of a well regulated militia".

In the 21st century, the amendment has been subjected to renewed academic inquiry and judicial interest. In District of Columbia v. Heller, the Supreme Court handed down a landmark decision that held the amendment protects an individual's right to keep a gun for self-defense. This was the first time the Court had ruled that the Second Amendment guarantees an individual's right to own a gun. In McDonald v. Chicago (2010), the Supreme Court clarified that the Due Process Clause of the Fourteenth Amendment incorporated the Second Amendment against state and local governments. In Caetano v. Massachusetts (2016), the Supreme Court reiterated its earlier rulings that "the Second Amendment extends, prima facie, to all instruments that constitute bearable arms, even those that were not in existence at the time of the founding" and that its protection is not limited to "only those weapons useful in warfare". The debate between various organizations regarding gun control and gun rights continues.