The Federal Security Service of the Russian Federation (FSB; Russian: Федеральная служба безопасности Российской Федерации (ФСБ), tr. Federal'naya sluzhba bezopasnosti Rossiyskoy Federatsii; IPA: ) is the principal security agency of Russia and the main successor agency to the USSR's Committee of State Security (KGB). Its main responsibilities are within the country and include counter-intelligence, internal and border security, counter-terrorism, and surveillance as well as investigating some other types of grave crimes and federal law violations. It is headquartered in Lubyanka Square, Moscow's centre, in the main building of the former KGB. The Director of the FSB since 2008 is army general Aleksandr Bortnikov.

The immediate predecessor of the FSB was the Federal Counterintelligence Service (FSK) of Russia, itself a successor to the KGB: on 12 April 1995, Russian president Boris Yeltsin signed a law mandating a reorganization of the FSK, which resulted in the creation of the FSB. In 2003, the FSB's responsibilities were widened by incorporating the previously independent Border Guard Service and a major part of the abolished Federal Agency of Government Communication and Information (FAPSI). The two major structural components of the former KGB that remain administratively independent of the FSB are the Foreign Intelligence Service (SVR) and the State Guards (FSO).

Under Russian federal law, the FSB is a military service just like the armed forces, the MVD, the FSO, the SVR, the FSKN and EMERCOM's civil defence, but its commissioned officers do not usually wear m