Abstract: This article explores the question of placement of the American anti-ballistic missile defense system THAAD in the Republic of Korea, reasons for deployment, as well as response of the Pacific Rim countries to deployment of anti-ballistic missile defense system. Particular attention is dedicated to Russia’s position on this problem. The object of this research is the bilateral relations between Russia and South Korea. The subject is the factor of placement of the American anti-ballistic missile defense system THAAD in the Republic of Korea and its impact upon the development of Russia-South Korea relations. The conclusion is made that the placement of the American anti-ballistic missile defense system THAAD in the Republic of Korea produced certain negative effect on the Russia-South Korea relations; however, there is no apparent deterioration in relations, the cooperation between the countries continues to develop, as well as Russian and South Korean leaders meet on the highest level. Nevertheless, there is a probability of deterioration in relations if the American anti-ballistic missile defense system in South Korea would pose a real threat to Russia.