The 173-page memoir was bought by a Japanese surgeon well-known for his right-wing views, who said he wished to restore it to the country's royal family.
"It should have been in Japan, but it ended up overseas," Dr. Katsuya Takasu told The Associated Press in Tokyo. "So it feels like it's finally coming back."
The memoir sold at the Bonhams auction house in Manhattan for $275,000 (€231,000).
Plastic surgeon Takasu, has been criticized by the Simon Wiesenthal Center, a Jewish human rights group, for using social media to praise Adolf Hitler and deny both the Holocaust and the 1937 Nanjing massacre.
Takasu hopes to be able to give the memoir to the only grandson of current Emperor Akihito, although there is a price limit on gifts to the imperial family of about $1,300.
Hirohito is said to have dictated the memoir to his aides in 1946, after the end of World War Two. It is thought to have been written at the request of General Douglas MacArthur, who was administering occupied Japan at the time.
The "imperial monologue" covers events beginning with the Japanese assassination of Manchurian warlord Zhang Zuolin in 1928 to the emperor's surrender broadcast recorded on August 14, 1945.
Civil conflict 'would have been worse'
In its conclusion, the memoir features a statement from the emperor that, if he had vetoed the decision to go to war, there would have been an even worse civil conflict and "Japan would have been destroyed."
The document is believed to be a carefully crafted text designed to play down Hirohito's responsibility in case he was prosecuted after the war.
The contents of its pages caused a sensation when they were first published in Japan in 1990, soon after the emperor's death.
Each of the two volumes are bound with string, with the contents written vertically in pencil. The memoir was transcribed by Hidenari Terasaki, an imperial aide and former diplomat who was translator when Hirohito met McArthur.
The transcript was kept by Terasaki's American wife Gwen Terasaki after his death in 1951.
rc/ng (AP, Reuters)