G-man (short for "government man", plural G-men) is an American slang term for agents of the United States Government. It is especially used as a term for an agent of the Federal Bureau of Investigation (FBI).
G-man is also a term used for members of G Division, a Dublin Metropolitan Police unit operating out of Dublin Castle prior to Irish independence in 1922. Colonel Ned Broy uses the term in his official testimony for the Irish Army's Bureau of Military History in their archive of the Easter Rising (1916) and the Irish War of Independence (1919–1921).