Internet censorship is the legal control or suppression of what can be accessed, published, or viewed on the Internet. Censorship is most often applied to specific internet domains (such as Wikipedia.org, for example) but exceptionally may extend to all Internet resources located outside the jurisdiction of the censoring state. Internet censorship may also put restrictions on what information can be made internet accessible. Organizations providing internet access – such as schools and libraries – may choose to preclude access to material that they consider undesirable, offensive, age-inappropriate or even illegal, and regard this as ethical behavior rather than censorship. Individuals and organizations may engage in self-censorship of material they publish, for moral, religious, or business reasons, to conform to societal norms, political views, due to intimidation, or out of fear of legal or other consequences.

The extent of Internet censorship varies on a country-to-country basis. While some countries have moderate Internet censorship, other countries go as far as to limit the access of information such as news and suppress and silence discussion among citizens. Internet censorship also occurs in response to or in anticipation of events such as elections, protests, and riots. An example is the increased censorship due to the events of the Arab Spring. Other types of censorship include the use of copyrights, defamation, harassment, and various obscene material claims as a way to deliberately suppress content.

Support for and opposition to Internet censorship also varies. In a 2012 Internet Society survey, 71% of respondents agreed that "censorship should exist in some form on the Internet". In the same survey, 83% agreed that "access to the Internet should be considered a basic human right" and 86% agreed that "freedom of expression should be guaranteed on the Internet". According to GlobalWebIndex, over 400 million people use virtual private networks to circumvent censorship or for increased user privacy.

Many of the challenges associated with Internet censorship are similar to those for offline censorship of more traditional media such as newspapers, magazines, books, music, radio, television, and film. One difference is that national borders are more permeable online: residents of a country that bans certain information can find it on websites hosted outside the country. Thus censors must work to prevent access to information even though they lack physical or legal control over the websites themselves. This in turn requires the use of technical censorship methods that are unique to the Internet, such as site blocking and content filtering.

Views about the feasibility and effectiveness of Internet censorship have evolved in parallel with the development of the Internet and censorship technologies:

Blocking and filtering can be based on relatively static blacklists or be determined more dynamically based on a real-time examination of the information being exchanged. Blacklists may be produced manually or automatically and are often not available to non-customers of the blocking software. Blocking or filtering can be done at a centralized national level, at a decentralized sub-national level, or at an institutional level, e.g., in libraries, universities or Internet cafés. Blocking and filtering may also vary within a country across different ISPs. Countries may filter sensitive content on an ongoing basis and/or introduce temporary filtering during key time periods such as elections. In some cases, the censoring authorities may surreptitiously block content to mislead the public into believing that censorship has not been applied. This is achieved by returning a fake "Not Found" error message when an attempt is made to access a blocked website.

Unless the censor has total control over all Internet-connected computers, such as in North Korea (who employs an intranet that only privileged citizens can access), or Cuba, total censorship of information is very difficult or impossible to achieve due to the underlying distributed technology of the Internet. Pseudonymity and data havens (such as Hyphanet) protect free speech using technologies that guarantee material cannot be removed and prevents the identification of authors. Technologically savvy users can often find ways to access blocked content. Nevertheless, blocking remains an effective means of limiting access to sensitive information for most users when censors, such as those in China, are able to devote significant resources to building and maintaining a comprehensive censorship system.

The term "splinternet" is sometimes used to describe the effects of national firewalls. The verb "rivercrab" colloquially refers to censorship of the Internet, particularly in Asia.