Japan has pledged $7.4 billion in aid over the next three years to help five Mekong states, in an attempt at fostering development in a resource-rich region also being courted by China.
"The stability and prosperity of East Asia will not be possible without the stability and prosperity of the Mekong region," Prime Minister Yoshihiko Noda told a joint press conference after the six-way summit in Tokyo on Saturday.
"The Japanese government will recognise the Mekong region as a significant destination of assistance and continue its cooperation strongly."
Noda met the five government leaders of Cambodia, Laos, Myanmar, Thailand and Vietnam, as Japan's export-dependent economy is on the lookout for cheap labour and investment to power growth.
The region along the lower reaches of the 4,800-kilometre (2,980-mile) Mekong River has historically been isolated by war and political turmoil and remains poorer than other parts of Southeast Asia.
Rivals Japan and China have for years poured aid and investment into the Mekong region, home to more than 220 million people, and are seen increasingly as competitors for influence.
The Saturday morning summit is the fourth of its kind, and will also provide the latest stage for Myanmar as it journeys back to international respectability.
During its years of isolation, Japan -- unlike its Western allies -- maintained trade ties and dialogue with Myanmar, warning that a hard line on the then-ruling junta could push it closer to China, its key ally and commercial partner.
Also Saturday, Noda will meet Myanmar President Thein Sein, when he is expected to announce that Japan is ready to forgive 300 billion yen ($3.7 billion) debt and restart a suspended assistance programme.