Ambassador Shingo Yamagami said Tokyo stood ready to co-operate on cutting-edge defence technology with the trilateral AUKUS alliance between Australia, the US and UK.
Mr Yamagami said Japan's place may not be obvious in an alliance between three English-speaking nations with a long history of military interoperability and integrated defence industries.
"At first glance, some may argue Japan has no skin in this game," the ambassador told the Advancing AUKUS conference on Monday.
"Why should AUKUS matter to Japan? AUKUS matters to us a lot."
He said the primary role of the agreement through which Australia will acquire nuclear-propelled submarines was to act as a deterrent in the Indo-Pacific.
This included the prospect of hosting Australia's nuclear submarines when the time came.
"We are a frontline state facing challenging circumstances in the dangerous neighbourhood of Southeast Asia," he said.
"Such submarines will increase regional deterrence."
He also spoke of Tokyo's increasing defence ties with Australia off the back of a significant agreement that will see troops from both nations train together more closely.
"This makes Australia Japan's most important defence and security alliance ... outside of the United States," Mr Yamagami said.
While not directly referring to China as a threat, Tokyo's envoy spoke of the need for alliances to manage a more dangerous region, noting Japan's future was tied to the West's force projection.
"In other words, what matters to you matters to us too," he said.
While cutting-edge technology such as hypersonic weapons isn't at the level where outside nations could be brought into the tent, Japan stood ready to offer its help, he said.
"Japan's co-operation with AUKUS holds great potential," Mr Yamagami said.
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