Park, who was in Vladivostok on Friday to attend the second annual Eastern Economic Forum, told Rossiya Segodnya, a government-owned news agency, the U.S. missile defense system is to be deployed to "protect the lives of" South Korean people.
The South Korean president also said the "essence of the problem" is North Korea's nuclear and missile threats.
"If the North Korean nuclear threat is removed, then naturally the need to deploy THAAD would also dissipate," Park said.
Park's statement suggested Seoul is willing to be flexible on THAAD within specific boundaries, and aimed to reassure the Russian public the missile defense system is not being placed to monitor other countries, according to South Korean newspaper Maeil Business.
Russia and China have voiced opposition to THAAD and have claimed the system's powerful radar may be used in regional surveillance.
Park also said there are no "practical benefits" to monitoring third-party countries, and Seoul has no plans to do so.
South Korean assurances may be driven partly by economic incentives.
Seoul is expected to conclude joint research on a free trade agreement between South Korea and the Eurasian Economic Union, or EAEU.
The South Korean press report did not include any statements from Park on whether North Korea would play any role in the potential FTA.