Federal officials and representatives of the U.S. Holocaust Memorial Museum made the announcement Wednesday.
The artifact is a 400-page diary kept from 1936 to 1944 by Alfred Rosenberg, a notorious leader of the Nazi Party in World War II who was privy to plans for establishment of the Nazi state, mass murder of Jews and planning of the war.
The "Rosenberg Diary" gives "details that one would never know about the politics within the top leadership of the Nazi party and the state," museum senior adviser Henry Mayer said.
Rosenberg was found guilty by the International Military Tribunal on charges including war crimes and crimes against humanity, and was hanged in 1946.
The diary was found in the possession of Dr. Robert Kempner, a former German lawyer who fled to the United States during the war and served as a prosecutor in the Nuremberg Trials.
Contrary to law and proper procedure, Kempner took the diary when he returned to his Lansdowne, Pa., home after the trials, and kept it until his 1993 death, an Immigration and Customs Enforcement statement said.