A statement at the close of the gathering on March 27 said it was the "fundamental responsibility" of all states to safeguard nuclear materials and keep them out of the hands of terrorists.
It added that "nuclear terrorism continues to be one of the most challenging threats to international security" and said countering that threat requires "strong national measures."
However, the statement provided no specific details on how governments intend to combat the threat.
The North Korean and Iranian nuclear programs dominated talks among leaders on the sidelines of the summit, but were not on the formal agenda of the three-day conference.
Need For 'Favorable Environment'
Officials from more than 50 nations were in the South Korean capital for the meeting.
U.S. President Barack Obama said the threat remains due to "bad actors" actively seeking unsecured nuclear materials.
"Of course, what is also undeniable is that the threat remains. There are still too many bad actors in search of these dangerous materials and these dangerous materials are still vulnerable in too many places," Obama told the summit.
South Korean President Lee Myung-bak said international cooperation was needed to "minimize highly enriched uranium and plutonium" getting into the wrong hands.
"We need to completely establish international cooperation to minimize highly enriched uranium and plutonium as well as to detect, trace, and respond to the illegal nuclear materials transaction. Also there should be significant progress in establishing an international norm," Lee said.
Chinese President Hu Jintao called for creating a "favorable international environment for strengthening nuclear security."
"[We must] eliminate the source of nuclear proliferation and nuclear terrorism, stick to the goals and principles of the UN Charter, stick to the new security concept that features mutual trust, mutual benefit, equality and cooperation, stick to using peaceful means to resolve hot issues and international disputes, and create a favorable international environment for strengthening nuclear security," Hu said.
Also at the summit, Obama joined Russian President Dmitry Medvedev and Kazakh Nursultan Nazarbaev in a joint statement unveiling details of the operation to secure a huge Soviet-era nuclear test site in Semey, formerly known as Semipalatinsk, in Kazakhstan, which was used for hundreds of Soviet nuclear tests between 1949 and 1989.
The first day of the summit was overshadowed by North Korea's plans to launch a satellite into space using a long-range rocket, which the West fears is a cover for nuclear missile development.
Obama warned North Korea and Iran the international community will not tolerate nuclear proliferation.
In turn, North Korea on March 28 vowed to go ahead with the launch despite the international condemnation. Official Korean Central News Agency quoted a North Korean Foreign Ministry spokesman as saying the North will “never give up the right to launch a peaceful satellite.” The statement also called on Obama to abandon his “confrontational” approach to North Korea.
The U.S. and its allies have warned that the satellite launch, planned for around mid-April, could be a pretext for a long-range missile test.
The United Nations Security Council has banned the North from conducting such tests as part of sanctions aimed at compelling the North to roll back its nuclear and missile programs.
Obama also vowed to pursue further nuclear arms cuts with Russia, saying the United States has more warheads than necessary.
The Seoul summit is a follow-up to the first such conference in Washington in 2010.
The Washington meeting came a year after Obama unveiled his vision of a world without nuclear weapons in a speech in the Czech capital, Prague.