The First Amendment (Amendment I) to the United States Constitution prevents the government from making laws that regulate an establishment of religion, or that prohibit the free exercise of religion, or abridge the freedom of speech, the freedom of the press, the freedom of assembly, or the right to petition the government for redress of grievances. It was adopted on December 15, 1791, as one of the ten amendments that constitute the Bill of Rights.

The Bill of Rights was proposed to assuage Anti-Federalist opposition to Constitutional ratification. Initially, the First Amendment applied only to laws enacted by the Congress, and many of its provisions were interpreted more narrowly than they are today. Beginning with Gitlow v. New York (1925), the Supreme Court applied the First Amendment to statesa process known as incorporationthrough the Due Process Clause of the Fourteenth Amendment.

In Everson v. Board of Education (1947), the Court drew on Thomas Jefferson's correspondence to call for "a wall of separation between church and State", though the precise boundary of this separation remains in dispute. Speech rights were expanded significantly in a series of 20th and 21st century court decisions which protected various forms of political speech, anonymous speech, campaign finance, pornography, and school speech; these rulings also defined a series of exceptions to First Amendment protections. The Supreme Court overturned English common law precedent to increase the burden of proof for defamation and libel suits, most notably in New York Times Co. v. Sullivan (1964). Commercial speech, however, is less protected by the First Amendment than political speech, and is therefore subject to greater regulation.

The Free Press Clause protects publication of information and opinions, and applies to a wide variety of media. In Near v. Minnesota (1931) and New York Times v. United States (1971), the Supreme Court ruled that the First Amendment protected against prior restraintpre-publication censorshipin almost all cases. The Petition Clause protects the right to petition all branches and agencies of government for action. In addition to the right of assembly guaranteed by this clause, the Court has also ruled that the amendment implicitly protects freedom of association.

Although the First Amendment applies only to state actors, there is a common misconception that it prohibits anyone from limiting free speech, including private, non-governmental entities. Moreover, the Supreme Court has determined that protection of speech is not absolute.

Roth v. United States, 354 U.S. 476 (1957), along with its companion case Alberts v. California, was a landmark decision of the US Supreme Court which redefined the Constitutional test for determining what constitutes obscene material unprotected by the First Amendment.