"I believe now is when close communication and cooperation between South Korea and Japan are more important than ever as we have the South-North Korea summit and the North-U.S. summit ahead, as well as the South Korea-Japan and China summit," the president said while meeting with Japanese Foreign Minister Taro Kono at his office Cheong Wa Dae.
Moon is set to meet the North Korean leader on April 27, which is expected to be followed by a meeting between U.S. President Donald Trump and Kim in May or early June.
The Moon-Kim summit, if held, will mark the third inter-Korean summit. The U.S.-North Korea summit will be the first of its kind in history.
Kono expressed his respect for what he called Moon's personal efforts to realize the upcoming summits with the reclusive North Korean leader.
"I pay my respect to the South Korean government's efforts toward the South-North summit and the U.S.-North Korea summit. I hope to work to increase cooperation between Japan and South Korea, as well as Japan, South Korea and the U.S., and realize the denuclearization of North Korea," he said through his interpreter.
The Japanese foreign minister arrived here Tuesday on a two-day visit for talks with his South Korean counterpart Kang Kyung-wha, which were held here earlier in the day.
Kono's trip marked the first of its kind by a Japanese foreign minister since December 2015.
The relationship between Seoul and Tokyo turned sour when the new South Korean administration began demanding an official apology from the Japanese government for Japan's World War II atrocities against its Asian neighbors, such as its sexual slavery of thousands of Korean women.
The Moon Jae-in administration has also been critical of a 2015 agreement signed by its conservative predecessor to settle the sexual slavery issue once and for all in exchange for 10 billion yen (US$9.3 million).
Moon has declared the deal seriously flawed, noting that many surviving victims of Japan's sexual slavery here have refused to accept any part of the money provided by the Japanese government, which was named a "support fund" and not legal compensation.
Still, the South Korean leader stressed the need to improve and even upgrade the countries' bilateral relationship.
"I hope the South Korea-Japan relationship will be further developed into the next stage and that the two countries will work together to that end," he told the visiting Japanese diplomat.
Kono too expressed hope for improve ties between the two countries, saying he also carried a special message from Japanese Prime Minister Shinzo Abe for the South Korean president, which was not disclosed.