The Ministry of State Security (MSS) is the principal civilian intelligence and security service of the People's Republic of China, responsible for foreign intelligence, counterintelligence, and political security of the Chinese Communist Party (CCP). One of the largest and most secretive intelligence organizations in the world, it maintains powerful semi-autonomous branches at the provincial, city, municipality and township levels throughout China. The ministry's headquarters, Yidongyuan, is a large compound in Beijing's Haidian district.

The origins of the MSS date to the beginnings of the CCP's Central Special Branch, replaced by the Central Committee Society Department from 1936 through the proclamation of the People's Republic in 1949. In 1955, the department was replaced with the Central Committee Investigation Department, which existed in various configurations through the Cultural Revolution to 1983, when it was merged with counterintelligence elements of the Ministry of Public Security (MPS) to form the MSS.

An executive department of the State Council, the contemporary MSS is an all-source intelligence organization with a broad mandate and expansive authorities to undertake global campaigns of espionage and covert action on the so-called "hidden front." Within China, the ministry leverages extrajudicial law enforcement authorities to achieve its domestic objectives: the State Security Police serve as a secret police authorized to detain and interrogate people in what is known as an "invitation to tea." Those remanded by state security are detained in the ministry's own detention facilities.

Outside the mainland, the ministry is best known for its numerous advanced persistent threat groups, some outsourced to contractors, which carry out prolific industrial and cyber espionage campaigns. The ministry has also been implicated in political and transnational repression and harassment of dissidents abroad. Its influence operations, carried out with the United Front Work Department in accordance with the "three warfares" doctrine, have produced some of the country's most pervasive diplomatic rhetoric including "great changes unseen in a century" and "China's peaceful rise." Estimates of the ministry's size range from 110,000 to 800,000 employees, with most of the workforce spread between the dozens of semi-autonomous bureaus across the country.

MSS functions as China's intelligence, security and secret police agency. A document from the U.S. Department of Justice described the agency as being like a combination of the Central Intelligence Agency (CIA) and Federal Bureau of Investigation (FBI). Australian author Clive Hamilton described it as being similar to an amalgamation of the Australian Security Intelligence Organisation (ASIO) and the Australian Secret Intelligence Service (ASIS) "with a lot more power and less subject to the constraints of the rule of law." It is an all-source intelligence organization with a broad mandate and expansive authorities to undertake global campaigns of espionage and covert action on the so-called "hidden front." Its influence operations have been carried out with the United Front Work Department in accordance with the "three warfares" doctrine.

According to Peter Mattis, president of the Jamestown Foundation and former CIA analyst, and his fellow analyst Matthew Brazil, a former U.S. Army officer and diplomat in Asia:

"The language Chinese intelligence uses reflects its Marxist–Leninist and revolutionary heritage. The lexicon suggests (as has been borne out in interviews with former officials who had routine contacts with their Chinese counterparts) that the intelligence services are bastions of faith in the CCP. Although they may be practical in terms of techniques and methods to acquire intelligence, this information is filtered through a Marxist–Leninist lens. The implication is that foreign targets are viewed in the worst possible light."

The MSS is a civilian agency that controls its own secret police force, the State Security Police, which is one of the four components of the People's Police. The State Security Police is authorized to detain and interrogate people in what is known as an "invitation to tea." Those remanded by state security are detained in the ministry's own detention facilities. The MSS seal contains the emblem of the Chinese Communist Party and the official uniform is identical to that of the other People's Police, with the only difference being the police insignia include the Chinese characters "国安" ("State Security").