The NSA spied on Japan's prime minister, central bank, finance ministry and major corporations, such as the natural gas division of Mitsubishi, according to documents released today.
The targets of the cyber-spying included stealing secrets on US-Japan relations, trade negotiations and climate change policy. Fruits of the spying, exposed in leaked documents published by WikiLeaks on Friday, were shared with the US's Five Eyes spying partners.
The apparently economic and politically-motivated spying started back during the administration of Prime Minister Shinzo Abe, which ran from September 2006 until September 2007.
It's unclear whether or not the spying remains ongoing, as is the source of the leaked documents.
In a statement, WikiLeaks Investigations Editor Sarah Harrison said: "The US government targeted sensitive Japanese industry and climate change policy. Would the effectiveness of Japan's industry and climate change proposals be different today if its communications had been protected?"