"A push by North Korea to become a nuclear state cannot be accepted or tolerated," Moon said in an address to parliament. "We also will not develop or own nuclear arms."
In recent months Pyongyang has carried out its sixth nuclear test - its most powerful by far - and launched missiles apparently capable of reaching much of the US mainland, raising concerns in Seoul about its security alliance with Washington.
South Korean media and opposition politicians have called for US tactical nuclear weapons, which were withdrawn from the peninsula in the 1990s, to be returned.
Some have suggested that if Washington does not agree Defence Secretary Jim Mattis expressed doubts about the concept in a visit at the weekend - Seoul should develop a nuclear capability of its own, in order to ensure what they dub a "balance of terror" on the peninsula.
But Moon said in his address that Seoul's approach would be "based on the joint declaration to denuclearise the Korean peninsula declared by both Koreas" in 1992.