Herbert Arthur Philbrick (May 11, 1915 – August 16, 1993) was a Boston-area advertising executive who was encouraged by the FBI to infiltrate the Communist Party USA between 1940 and 1949.

Shortly thereafter, in July 1945, as a result of the Duclos letter—a letter by a leading French Communist, which actually was a policy directive that originated in Moscow—the Party turned away from Browderism and again took a Marxist–Leninist line, though not completely abandoning the tactics of the united front.

His testimony was perhaps most useful in that he demonstrated from the content of the training which he had received that the intent of the Communist Party was to overthrow the government of the United States.

[6][7] In addition, a television series called I Led 3 Lives, starring Richard Carlson and Ed Hinton, loosely based on Philbrick's experiences, aired in syndication for three seasons during the 1950s.

[4] Later in life, Philbrick retired to the home of his youth, in the Little Boar's Head district of North Hampton, New Hampshire.

He remained active, giving speeches and encouraging youth and adult citizens to exercise their political rights and power, admonishing his listeners to be ever-watchful against those who would undermine the republican form of government.

[9] Philbrick was father to six children with his first wife Eva: Dale, Brenda, Leslie, Connie, Sandra, and Herbert Jr.