Japanese government sources say Prime Minister Kishida Fumio and China's Premier Li Qiang had brief contact on the sidelines of ASEAN meetings in Indonesia.
The sources say the two met on Wednesday shortly before a summit between ASEAN, Japan, China and South Korea.
China has been strongly opposed to Japan's release of treated and diluted water from the crippled Fukushima Daiichi nuclear power plant into the ocean.
It remains unclear whether Kishida and Li had a conversation about the issue.
The Fukushima Daiichi plant suffered a triple meltdown in the 2011 earthquake and tsunami. Water used to cool molten fuel at the plant has been mixing with rain and groundwater.
The accumulated water is treated to remove most radioactive substances, but still contains tritium.
Before releasing the treated water into the sea, the Tokyo Electric Power Company dilutes it to reduce tritium levels to about one-seventh of the World Health Organization's guidance level for drinking water.