U.S. counterintelligence agent for the Federal Bureau of Investigation (FBI) who spied on the Communist Party of the United States during the 1940s.

Philbrick studied engineering at Lincoln Technical Institute of Northeastern University in Boston, and in 1938 he became an advertising salesman. Through a sales call, he became interested in the Massachusetts Youth Council and later helped to set up a subsidiary organization in Cambridge. Gradually he came to realize that the organization was controlled and used for propaganda by the Communist Party. He took his suspicions to the FBI and was asked by them to act as an undercover agent. He did so for nine years, reporting on activities that violated the Smith Act and other laws. On April 6, 1949, he broke cover to testify against 11 Communist leaders who had been indicted in part on evidence he had provided. All 11 were found guilty, and Philbrick became a public figure. In 1952 he published a record of his undercover work called I Led Three Lives, which became a best-seller.