"These 400 pages are a window into the dark soul of one of the great wrongs in human history," John Morton, director of U.S. Immigration and Customs Enforcement, said at a news conference announcing the find.
Agents from Homeland Security Investigations seized the diary in April from a home in Amherst, outside Buffalo.
They declined to discuss many details of their investigation - including who last had the document - but acknowledged tracking the diary to a friend of a secretary who once worked for Robert Kempner, a U.S. war-crimes prosecutor.
The world knew about the diary, which covers Rosenberg's life from 1934 to 1944, because prosecutors cited parts of it during the Nuremberg trials, in which Rosenberg was convicted and then executed in 1946.
But the complete document vanished after the trials, and was believed to have been among thousands of pieces of evidence Kempner smuggled out of Germany.
Rosenberg was a chief proponent of the racial-purity policies that led to the Holocaust. As the administrator for occupied lands on the Eastern Front, he played "a significant role" in the mass enslavement and murder of Jews in the Baltic states, Ukraine, and what is now Belarus, historians say.
In one March 1941 passage, Rosenberg wrote of having just returned from a "successful" conference on a program to rid Europe of Jews.
"Reading Rosenberg's diary is to stare into the mind of a dark soul," Morton said.
Archivists have not fully translated the document, but hope it may shed new light on the Nazi war effort. Two months after Hitler proclaimed that his "hour" had come, Rosenberg oversaw Operation Barbarossa, the invasion of Russia.
Kempner, by contrast, was a German-born Jew who fled his homeland for the United States in the 1930s and returned after the war as an American prosecutor.
Until his death in 1993, he lived at times in a house in Lansdowne, Delaware County, and his sons had agreed to turn over his possessions to the U.S. Holocaust Memorial Museum. But despite recouping thousands of documents, the archivists couldn't find Rosenberg's diary.
Their hopes rose again in 1999, when a Folcroft man who had been hired to clean out Kempner's vacant Lansdowne house found 40 more file boxes of documents.
Those materials included German papers outlining the Nazis' "aggressive war against and the plundering, spoliation, and the economic exploitation of the Soviet Union by the Nazi regime," according to a filing at the time by prosecutors in Philadelphia.
But the diary was still missing.
"That was what we were looking for. That's what was so frustrating," said Robert Wittman, a former FBI agent who specialized in art crimes and spent more than a decade helping in the search.
Henry Mayer, a senior adviser on archives at the museum, said a breakthrough came when a German reporter told him that Kempner's onetime secretary, Margot Lipton, had given the diary to a friend "for safekeeping" before her death.
On April 5, agents bearing a warrant found it at that friend's house in Amherst.
Morton, who announced the discovery with U.S. Attorney Charles Oberly III, declined to name the man or say if anyone faced criminal charges. He said the man voluntarily turned over the document.
Officials displayed a half-dozen or so yellowed pages from the diary at the news conference in the Wilmington office of Homeland Security Investigations, the lead agency in the seizure. The materials will eventually be given to Holocaust Memorial Museum in Washington.
Morton and Mayer credited a list of law enforcement agencies and agents for staying on the case over the years, including police in Lansdowne and New York, HSI and the Wilmington-based prosecutor, Assistant U.S. Attorney David Hall.
There are no copies, Morton said, so the diary is invaluable.
"As time marches on, there are fewer and fewer living victims and witnesses to the horror of the Third Reich," he said. "Soon there will be no more human testimony of what happened during the Holocaust and, just as important, how it happened."
Contact John P. Martin at 215-925-2649, at email@example.com or @JPMartinInky on Twitter.