His consultancy confirmed on Friday that he died late last month, aged 74.
After retiring from the foreign ministry in 1991, Okamoto served as an adviser to then-prime minister Ryutaro Hashimoto on issues related to Okinawa, home to the bulk of U.S. troops in Japan.
He also advised then-prime minister Junichiro Koizumi from 2001-2004.
He was also a research fellow at the Massachusetts Institute of Technology’s Center for International Studies.
Former U.S. defence official Richard Armitage called Okamoto a “giant” among those involved in U.S.-Japan affairs and a true patriot.
“This is a shock. And from the coronavirus!” said Tokyo Governor Yuriko Koike in a tweet.
AFRICA DAILY NEWS, NEW YORK