"The Republic of Korea's achievement of democracy and prosperity stands in stark contrast to the tyranny and poverty across the border to the North," she said.
Washington would not change its policy on Pyongyang as long as it continued "insulting and refusing dialogue" with Seoul, she added.
Praising South Korea's position, Clinton said Seoul had shown "calm resolve and determination" in facing "provocative and unhelpful statements and actions" from the North.
Clinton later announced the appointment of Stephen Bosworth, a career diplomat, as the new US envoy to replace Christopher Hill to six-nation talks on ending the North's nuclear programme.
"The Republic of Korea's achievement of democracy and prosperity stands in stark contrast to the tyranny and poverty across the border to the North"
US secretary of state
"[We will] try to convince the North Koreans to begin a process within the six-party talks toward the complete and verifiable elimination of nuclear weapons," she said.
Bosworth served as US ambassador to South Korea between 1997 and 2000 – a period that coincided with warming ties between North and South Korea and the build-up to the historic summit between the leaders of the two countries.
Clinton said a renewed US effort in the six-party talks would determine the best way to approach North Korea by presenting a "united front" to continue pushing for nuclear disarmament.
On Thursday, the North accused the US of planning a nuclear attack and said it was ready for war with South Korea.
Recent weeks have seen tensions escalate on the Korean peninsula following increasingly angry rhetoric by the North and reports that it may be preparing to test its longest-range Taepodong 2 missile.
Experts say the missile is designed to reach as far as the US west coast, but has never successfully flown.
In warning North Korea against violating a UN Security Council resolution to suspend all activities related to its ballistic missile programme, Clinton said any move to proceed with the reported missile launch could harm nuclear talks and "aggravate tensions in the region".
Clinton held talks with senior South Korean officials before she travelled to China on the last leg of her regional tour.