The Sherman Antitrust Act of 1890 (26 Stat. 209, 15 U.S.C. §§ 1–7) is a United States antitrust law which prescribes the rule of free competition among those engaged in commerce and consequently prohibits unfair monopolies. It was passed by Congress and is named for Senator John Sherman, its principal author.

The Sherman Act broadly prohibits 1) anticompetitive agreements and 2) unilateral conduct that monopolizes or attempts to monopolize the relevant market. The Act authorizes the Department of Justice to bring suits to enjoin (i.e. prohibit) conduct violating the Act, and additionally authorizes private parties injured by conduct violating the Act to bring suits for treble damages (i.e. three times as much money in damages as the violation cost them). Over time, the federal courts have developed a body of law under the Sherman Act making certain types of anticompetitive conduct per se illegal, and subjecting other types of conduct to case-by-case analysis regarding whether the conduct unreasonably restrains trade.

The law attempts to prevent the artificial raising of prices by restriction of trade or supply. "Innocent monopoly", or monopoly achieved solely by merit, is legal, but acts by a monopolist to artificially preserve that status, or nefarious dealings to create a monopoly, are not. The purpose of the Sherman Act is not to protect competitors from harm from legitimately successful businesses, nor to prevent businesses from gaining honest profits from consumers, but rather to preserve a competitive marketplace to protect consumers from abuses.

In Spectrum Sports, Inc. v. McQuillan 506 U.S. 447 (1993) the Supreme Court said:

The purpose of the [Sherman] Act is not to protect businesses from the working of the market; it is to protect the public from the failure of the market. The law directs itself not against conduct which is competitive, even severely so, but against conduct which unfairly tends to destroy competition itself.

According to its authors, it was not intended to impact market gains obtained by honest means, by benefiting the consumers more than the competitors. Senator George Hoar of Massachusetts, another author of the Sherman Act, stated that "a man who merely by superior skill and intelligence [...] got the whole business because nobody could do it as well as he could was not a monopolist," but that it was monopoly if their business practices "made it impossible for other persons to engage in fair competition".

In 1937, the business Apex Hosiery suffered losses when strikers occupied their business, and prevented shipments of orders, including shipments to other states. Apex attempted to sue the union leader for treble damages as provided by the Sherman Act. Apex argued that the union had interfered with interstate commerce. The court held that the Sherman Act did not apply to the unlawful occupation of a business.^{[citation needed]}

The Supreme Court stated in the case Apex Hosiery Co. v. Leader 310 U.S. 469, 310 U. S. 492-93 and n. 15: