The Great Firewall (GFW; simplified Chinese: 防火长城; traditional Chinese: 防火長城; pinyin: Fánghuǒ Chángchéng) is the combination of legislative actions and technologies enforced by the People's Republic of China to regulate the Internet domestically. Its role in internet censorship in China is to block access to selected foreign websites and to slow down cross-border internet traffic. The Great Firewall operates by checking transmission control protocol (TCP) packets for keywords or sensitive words. If the keywords or sensitive words appear in the TCP packets, access will be closed. If one link is closed, more links from the same machine will be blocked by the Great Firewall. The effect includes: limiting access to foreign information sources, blocking popular foreign websites (e.g. Google Search, Facebook, Twitter, Wikipedia, and others) and mobile apps, and requiring foreign companies to adapt to domestic regulations.

Besides censorship, the Great Firewall has also influenced the development of China's internal internet economy by giving preference to domestic companies and reducing the effectiveness of products from foreign internet companies. The techniques deployed by the Chinese government to maintain control of the Great Firewall can include modifying search results for terms, such as they did following Ai Weiwei’s arrest, and petitioning global conglomerates to remove content, as happened when they petitioned Apple to remove the Quartz business news publication’s app from its Chinese App Store after reporting on the 2019–2020 Hong Kong protests.

The Great Firewall was formerly operated by the SIIO, as part of the Golden Shield Project. Since 2013, the firewall is technically operated by the Cyberspace Administration of China (CAC), which is the entity in charge of translating the Chinese Communist Party's ideology and policy into technical specifications.

As mentioned in the "one country, two systems" principle, China's special administrative regions (SARs)—Hong Kong and Macau—are not affected by the firewall, as SARs have their own governmental and legal systems and therefore enjoy a higher degree of autonomy. Nevertheless, the U.S. State Department has reported that the central government authorities have closely monitored Internet use in these regions, and Hong Kong's National Security Law has been used to block websites documenting anti-government protests.

Provincial governments in parts of China, such as Henan Province, run their own versions of the firewall.

The term Great Firewall of China is a combination of the word firewall with the Great Wall of China. The phrase "Great Firewall of China" was first used in print by Australian sinologist Geremie Barmé in 1997.

A favorite saying of Deng Xiaoping's in the early 1980s, "If you open the window, both fresh air and flies will be blown in", is considered to be the political and ideological basis of the GFW Project. The saying is related to a period of the economic reform of China that became known as the "socialist market economy". Superseding the political ideologies of the Cultural Revolution, the reform led China towards a market economy and opened up the market for foreign investors. Nonetheless, despite the economic freedom, values and political ideas of the Chinese Communist Party (CCP) have had to be protected by "swatting flies" of other unwanted ideologies.

The internet in China arrived in 1994, as the inevitable consequence of and supporting tool for a "socialist market economy". Gradually, while Internet availability has been increasing, the Internet has become a common communication platform and tool for trading information.