Internal Security Department (Singapore)

The Internal Security Department (ISD) is the principal security agency and domestic intelligence service of Singapore. The department is tasked with collecting and analysing intelligence, making assessments, and taking executive actions to counter national security threats to the country's sovereignty, safety, and stability. As a counter-intelligence and counter-terrorism organisation, the ISD is responsible for protecting Singapore from espionage or spying, foreign interference, covert operations, subversion, terrorism, organised crime, and political, racial or religious extremism.

Commenting on the battle against the Communist Party of Malaya (CPM), then Deputy Prime Minister Goh Keng Swee stated that an "efficient secret police" was necessary to counter dangers such as insurgencies and violent rebellions in the country. The ISD is empowered to conduct mass surveillance and covert security operations. It also has the utmost right to indefinitely detain individuals without trial, when a suspect is believed to be a threat to Singapore's national security.

Although the agency falls under the Ministry of Home Affairs (MHA), it is autonomous within the ministry. It is led by a director, who holds the rank equivalent to a permanent secretary, and reports directly to the Prime Minister's Office (PMO). The department is highly secretive; most of its personnel are only known to the country's top government officials.