The Second Amendment (Amendment II) to the United States Constitution protects the right of the people to keep and bear arms and was adopted on December 15, 1791, as part of the first ten amendments contained in the Bill of Rights. The Supreme Court of the United States has ruled that the right belongs to individuals, while also ruling that the right is not unlimited and does not prohibit all regulation of either firearms or similar devices. State and local governments are limited to the same extent as the federal government from infringing this right, per the incorporation of the Bill of Rights.

The most influential framer of the 2nd 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 confidently contrasted the federal government of the United States to the European kingdoms, which he contemptuously described as "afraid to trust the people with arms." He assured his fellow citizens that they need never fear their government because "besides the advantage of being armed, which the Americans possess over the people of almost every other nation, the existence of subordinate governments, to which the people are attached, and by which the militia officers are appointed, forms a barrier against the enterprises of ambition".

In United States v. Cruikshank (1876), the Supreme Court of the United States 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" and limited the scope of the Second Amendment's protections to the federal 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 twenty-first century, the amendment has been subjected to renewed academic inquiry and judicial interest. In District of Columbia v. Heller (2008), the Supreme Court handed down a landmark decision that held the amendment protects an individual right to possess and carry firearms. In McDonald v. Chicago (2010), the Court clarified its earlier decisions that limited the amendment's impact to a restriction on the federal government, expressly holding that the Due Process Clause of the Fourteenth Amendment incorporates 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".