Moon campaigned on a program of cutting South Korea's traditional reliance on coal and nuclear for the bulk of its power, but has not previously commented on the commitment to end nuclear power since being elected in early May.
"We will end the nuclear-oriented power generation plan and pave the way for a nuclear-free era," Moon said at an event marking the closure of the Kori No.1 nuclear reactor in Busan, some 300 km (186 miles) southeast of Seoul.
"We will withdraw existing plans to build new nuclear power plants and not extend the lifespan of nuclear power plants."
South Korea's oldest nuclear reactor Kori No.1 was permanently shut down at midnight on Sunday after reaching the end of its 40-year-lifespan, the first South Korean nuclear power plants to be closed permanently.
South Korea has 25 nuclear reactors, supplying about a third of the country's total electricity. During his campaign, Moon vowed to review plans to add new eight nuclear reactors, including the part-completed Shin Kori No.5 and Kori No.6.
He also said the government will seek to shut down the country's second-oldest nuclear reactor, the Wolsong No.1, as soon as possible depending on the country's power supply conditions.
Public support for nuclear power has been undermined by a local scandal in 2010 over forged certificates for spare parts and the 2011 Fukushima meltdown in neighbouring Japan.
South Korea is seeking to scale back reliance on nuclear, and Moon has said he will support renewables and liquefied natural gas sectors in a bid to boost clean and safe energy.
The new government plans to increase the use of renewables to 20 percent of the country's total power generation by 2030.
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