The Stop Online Piracy Act (SOPA) is a United States bill introduced by U.S. Representative Lamar S. Smith (R-TX) to expand the ability of U.S. law enforcement to combat online copyright infringement and online trafficking in counterfeit goods. Provisions include the requesting of court orders to bar advertising networks and payment facilities from conducting business with infringing websites, and search engines from linking to the websites, and court orders requiring Internet service providers to block access to the websites. The law would expand existing criminal laws to include unauthorized streaming of copyrighted content, imposing a maximum penalty of five years in prison.

Proponents of the legislation state it will protect the intellectual-property market and corresponding industry, jobs and revenue, and is necessary to bolster enforcement of copyright laws, especially against foreign-owned and operated websites. Claiming flaws in present laws that do not cover foreign-owned and operated websites, and citing examples of active promotion of rogue websites by U.S. search engines, proponents assert stronger enforcement tools are needed.

Opponents state the proposed legislation threatens free speech and innovation, and enables law enforcement to block access to entire internet domains due to infringing content posted on a single blog or webpage. They have raised concerns that SOPA would bypass the "safe harbor" protections from liability presently afforded to websites by the Digital Millennium Copyright Act. Library associations have expressed concerns that the legislation's emphasis on stronger copyright enforcement would expose libraries to prosecution. Other opponents state that requiring search engines to delete domain names violates the First Amendment and could begin a worldwide arms race of unprecedented Internet censorship.

On January 18, 2012, the English Wikipedia, Reddit, and an estimated 7,000 other smaller websites coordinated a service blackout, to raise awareness. In excess of 160 million people viewed Wikipedia's banner. Other protests against SOPA and PIPA included petition drives, with Google stating it collected over 7 million signatures, boycotts of companies and organizations that support the legislation, and an opposition rally held in New York City.

In response to the protest actions, the Recording Industry Association of America (RIAA) stated, "It's a dangerous and troubling development when the platforms that serve as gateways to information intentionally skew the facts to incite their users and arm them with misinformation", and "it's very difficult to counter the misinformation when the disseminators also own the platform. "

Access to websites of several pro-SOPA organizations and companies such as RIAA, CBS.com, and others was impeded or blocked with denial of service attacks which started on January 19. Self-proclaimed members of the "hacktivist" group Anonymous claimed responsibility and stated the attacks were a protest of both SOPA and the United States Department of Justice's shutdown of Megaupload on that same day.

Opponents of the bill have proposed the Online Protection and Enforcement of Digital Trade Act (OPEN) as an alternative. On January 20, 2012, House Judiciary Committee Chairman Smith postponed plans to draft the bill: "The committee remains committed to finding a solution to the problem of online piracy that protects American intellectual property and innovation... The House Judiciary Committee will postpone consideration of the legislation until there is wider agreement on a solution. "