Censorship in the United States: The U.S. Congress enacts the Comstock Law, making it illegal to send any "obscene, lewd, or lascivious" books through the mail.

The Comstock Laws were a set of federal acts passed by the United States Congress under the Grant administration along with related state laws. The "parent" act (Sect. 211) was passed on March 3, 1873, as the Act for the Suppression of Trade in, and Circulation of, Obscene Literature and Articles of Immoral Use. This Act criminalized any use of the U.S. Postal Service to send any of the following items: obscenity, contraceptives, abortifacients, sex toys, personal letters with any sexual content or information, or any information regarding the above items.

A similar federal act (Sect. 245) of 1909 applied to delivery by interstate "express" or any other common carrier (such as railroad), rather than delivery by the U.S. Post Office. In addition to these federal laws, about half of the states enacted laws related to the federal Comstock laws. These state laws are considered by women's rights activist Mary Dennett to also be "Comstock laws". The laws were named after their chief proponent, US Postal Inspector and anti-vice activist Anthony Comstock. Comstock received a commission from the Postmaster General to serve as a special agent for the U.S. Post Office Department.In Washington, D.C., where the federal government had direct jurisdiction, another Comstock act (Sect. 312) also made it illegal (punishable by up to five years at hard labor), to sell, lend, or give away any "obscene" publication, or article used for contraception or abortion. Section 305 of the Tariff Act of 1922 forbade the importation of any contraceptive information or means.Numerous failed attempts were made to repeal or modify these laws and eventually, many of them (or portions of them) were declared unconstitutional. In a 1919 issue of the Journal of Criminal Law & Criminology, Judge J. C. Ruppenthal, after reviewing the various laws (especially state laws) called the set of acts "haphazard and capricious" and lacking "any clear, broad, well-defined principle or purpose".The restrictions on birth control in the Comstock laws were effectively rendered null and void by Supreme Court decisions Griswold v. Connecticut (1965) and Eisenstadt v. Baird (1972).

Censorship in the United States involves the suppression of speech or public communication and raises issues of freedom of speech, which is protected by the First Amendment to the United States Constitution. Interpretation of this fundamental freedom has varied since its enshrinement. For instance, restraints increased during the 1950s period of widespread anti-communist sentiment, as exemplified by the hearings of the House Committee on Un-American Activities. In Miller v. California (1973), the U.S. Supreme Court found that the First Amendment's freedom of speech does not apply to obscenity, which can, therefore, be censored. While certain forms of hate speech are legal so long as they do not turn to action or incite others to commit illegal acts, more severe forms have led to people or groups (such as the Ku Klux Klan) being denied marching permits or the Westboro Baptist Church being sued, although the initial adverse ruling against the latter was later overturned on appeal to the U.S. Supreme Court case Snyder v. Phelps.

The First Amendment protects against censorship imposed by law, but does not protect against corporate censorship, the restraint of speech of spokespersons, employees, or business associates by threatening monetary loss, loss of employment, or loss of access to the marketplace. Legal expenses can be a significant hidden restraint where there is fear of suit for libel. Many people in the United States are in favor of restricting censorship by corporations, citing a slippery slope that if corporations do not follow the Bill of Rights, the government will be influenced.Analysts from Reporters Without Borders ranked the United States 44th in the world out of 180 countries in their 2021 Press Freedom Index. Certain forms of speech, such as obscenity and defamation, are restricted in communications media by the government or by the industry on its own.