In the United States, antitrust law is a collection of mostly federal laws that govern the conduct and organization of businesses in order to promote economic competition and prevent unjustified monopolies. The three main U.S. antitrust statutes are the Sherman Act of 1890, the Clayton Act of 1914, and the Federal Trade Commission Act of 1914. Section 1 of the Sherman Act prohibits price fixing and the operation of cartels, and prohibits other collusive practices that unreasonably restrain trade. Section 2 of the Sherman Act prohibits monopolization. Section 7 of the Clayton Act restricts the mergers and acquisitions of organizations that may substantially lessen competition or tend to create a monopoly. The Robinson–Patman Act, an amendment to the Clayton Act, prohibits price discrimination.

Federal antitrust laws provide for both civil and criminal enforcement. Civil antitrust enforcement occurs through lawsuits filed by the Federal Trade Commission (FTC), the Antitrust Division of the U.S. Department of Justice, and private parties who have been harmed by an antitrust violation. Criminal antitrust enforcement is done only by the Justice Department's Antitrust Division. Additionally, U.S. state governments may also enforce their own antitrust laws, which mostly mirror federal antitrust laws, regarding commerce occurring solely within their own state's borders.

The scope of antitrust laws, and the degree to which they should interfere in an enterprise's freedom to conduct business, or to protect smaller businesses, communities and consumers, are strongly debated. Some economists argue that antitrust laws actually impede competition, and may discourage businesses from pursuing activities that would be beneficial to society. One view suggests that antitrust laws should focus solely on the benefits to consumers and overall efficiency, while a broad range of legal and economic theory sees the role of antitrust laws as also controlling economic power in the public interest.

Surveys of American Economic Association (AEA) members since the 1970s have shown that professional economists generally agree with the statement: "Antitrust laws should be enforced vigorously." A 1990 survey of AEA members found that 72 percent generally agreed that "Collusive behavior is likely among large firms in the United States", while a 2021 survey found that 85 percent generally agreed that "Corporate economic power has become too concentrated."

In the United States and Canada, and to a lesser extent in the European Union, the modern law governing monopolies and economic competition is known by its original name — "antitrust law". The term "antitrust" came from late 19th-century American industrialists' practice of using trusts—legal arrangements where one is given ownership of property to hold solely for another's benefit—to consolidate separate companies into large conglomerates. These "corporate trusts" died out in the early 20th century as U.S. states passed laws that made it easier to create new corporations. In most other countries, antitrust law is now called "competition law" or "anti-monopoly law".

American antitrust law formally began in 1890 when the U.S. Congress passed the Sherman Act, though a few U.S. states had passed local antitrust laws during the previous year. Using broad and general wording, the Sherman Act outlawed "monopoliz[ation]" and "every contract, combination ... or conspiracy in restraint of trade".

Every contract, combination in the form of trust or otherwise, or conspiracy, in restraint of trade or commerce among the several States, or with foreign nations, is declared to be illegal. Every person who shall make any contract or engage in any combination or conspiracy hereby declared to be illegal shall be deemed guilty of a felony ....

— Sherman Act, Section 1 (15 U.S.C. § 1).