Approved two years ago by the Japanese government and green-lighted by the United Nations nuclear watchdog last month, the discharge is a key step in a dauntingly long and difficult process of decommissioning the Fukushima Daiichi plant, including the removal of molten fuel after it was destroyed by a tsunami.
Plant operator Tokyo Electric Power (Tepco) said the release began at 1.03pm local time (0403 GMT) on Thursday and it had not identified any abnormalities with the seawater pump or surrounding facilities.
However, China reiterated on Thursday its firm opposition to the plan and said the Japanese government had not proved the legitimacy of the water discharge.
"The Japanese side should not cause secondary harm to the local people and even the people of the world out of its own selfish interests," China's foreign ministry said in a statement.
China has vowed to take measures to protect the marine environment and public health and step up monitoring of radiation levels in its waters following the discharge.
Tokyo has in turn criticised China for spreading "scientifically unfounded claims".
It maintains the water release is safe, noting that the International Atomic Energy Agency (IAEA) has also concluded that the impact it would have on people and the environment was "negligible".
The Fukushima Daiichi plant was destroyed in March 2011 after a massive 9.0 magnitude earthquake generated powerful tsunami waves that caused the meltdowns of three of its reactors.
The release of the wastewater has unsettled other countries in the region, with Cook Islands Prime Minister Mark Brown saying that while science supported Japan's decision, the region might not agree on the "complex" issue.
Japanese fishing groups, hit with years of reputational damage from radiation fears, have long opposed the plan.
They fear it will lead to a loss of sales, including from export restrictions to major markets.
Hong Kong and Macau - both Chinese-ruled regions - are set to implement a ban on Japanese seafood from regions including the capital Tokyo and Fukushima from Thursday.
South Korean Prime Minister Han Duck-soo said import bans on Fukushima fisheries and food products would stay in place until public concerns were eased.
The water will be released in smaller portions initially and with extra checks.
The first discharge totalling 7800 cubic metres - the equivalent of about three Olympic swimming pools of water - will take place across about 17 days.
According to Tepco test results released on Thursday, that water contains about up to 63 becquerels of tritium per litre, below the World Health Organisation drinking water limit of 10,000 becquerels per litre.
A becquerel is a unit of radioactivity.
Japan will conduct monitoring around the water release area and publish results weekly, the environment minister said.
Tepco expects the process of releasing the wastewater - currently totalling more than 1.3 million metric tons - to take about 30 years.
Civic groups have launched protests in Japan and South Korea, although South Korea's government has said its own assessment found no problems with the scientific and technical aspects of the release.
South Korean police arrested at least 14 protesters who entered the Japanese embassy in Seoul, according to an organiser and a Reuters witness.
