Japan was a country before the Great War, located in Asia.
Almost nothing is known about the country after the Great War.
In 1941, the Empire of Japan attacks the United States of America at Pearl Harbor. One year later the Sierra Army Depot was constructed to defend the United States should Japan attack the west coast. In 1945 the United States military dropped two atomic bombs on the Japanese cities of Hiroshima and Nagasaki, the bombs being named "Little Boy" and "Fat Man" respectively. This made Japan the only country to have been hit by nuclear weapons before the Great War.
In 2241, some descendants of the Yakuza, the pre-War Japanese mafia, are around New Reno, perpetuating the tradition of their expertise in swordplay and throwing weapons. They have no known headquarters and travel in small bands, fighting against New Reno's mobsters and scouting the Wasteland around New Reno. They never succeed in entering into the city, however.
In 2277, Toshiro Kago, a samurai of the Oda clan born sometime before 1562 during the Sengoku period, was found aboard mothership Zeta by the Lone Wanderer after being abducted by aliens seven centuries earlier.
Japan is mentioned in the history of Sierra Army Depot in Fallout 2, although Yakuza and the wakizashi appeared in Fallout 2, and Toshiro Kago, with a katana, okegawa dô and the kabuto appeared in the Fallout 3 add-on Mothership Zeta. The Katana appears in the Fallout: New Vegas add-on Gun Runners' Arsenal.