Robert Philip Hanssen (born April 18, 1944) is an American former Federal Bureau of Investigation (FBI) double agent who spied for Soviet and Russian intelligence services against the United States from 1979 to 2001. His espionage was described by the Department of Justice as "possibly the worst intelligence disaster in U.S. history." Hanssen is currently serving 15 consecutive life sentences without parole at ADX Florence, a federal supermax prison near Florence, Colorado.

In 1979, three years after joining the FBI, Hanssen approached the Soviet Main Intelligence Directorate (GRU) to offer his services, launching his first espionage cycle, which lasted until 1981. He restarted his espionage activities in 1985 and continued until 1991 when he broke off communications during the collapse of the Soviet Union, fearing he would be exposed. Hanssen restored communications the next year and continued until his arrest. Throughout his spying, Hanssen remained anonymous to the Russians.

Hanssen sold thousands of classified documents to the KGB that detailed U.S. strategies in the event of nuclear war, developments in military weapons technologies, and aspects of the U.S. counterintelligence program. He was spying at the same time as was Aldrich Ames in the Central Intelligence Agency (CIA). Both Ames and Hanssen compromised the names of KGB agents working secretly for the U.S., some of whom were executed for their betrayal. Hanssen also revealed a multimillion-dollar eavesdropping tunnel built by the FBI under the Soviet Embassy. After Ames's arrest in 1994, some of these intelligence breaches still remained unsolved. The FBI paid $7 million to a KGB agent to obtain a file on an anonymous mole, whom the FBI later identified as Hanssen through fingerprint and voice analysis.

Hanssen was arrested on February 18, 2001, at Foxstone Park near his home in the Washington, D.C., suburb of Vienna, Virginia, after leaving a package of classified materials at a dead drop site. He was charged with selling U.S. intelligence documents to the Soviet Union and subsequently Russia for more than $1.4 million in cash and diamonds over a 22-year period. To avoid the death penalty, Hanssen pleaded guilty to 14 counts of espionage and one of conspiracy to commit espionage. He was sentenced to 15 life terms without the possibility of parole.