This paper analyzes South Korea's China policy since the Korean War and discusses the implications of evolving Sino–ROK relations for East Asian security. South Korea's engagement policy toward China since their diplomatic normalization in 1992 has been so successful that the Chinese government now describes the two countries' relationship as “a matured strategic cooperative partnership,” which is the second highest in the pecking order of China's diplomatic relations. China has quickly become South Korea's largest trading partner, replacing the United States. Sino–ROK political and social ties have also deepened following their robust economic ties. However, growing Sino–ROK ties have significant implications for East Asian security, as South Korea is a key US ally, whereas China is North Korea's main ally and patron. This paper contends that South Korea's relations with the United States and China should not be viewed as a zero‐sum game. Seoul will not sever its close security ties with Washington due to its strategic partnership with Beijing, just as Beijing would not abandon Pyongyang because of its strong economic ties with Seoul. Given Beijing's persistent support for the survival of Pyongyang's regime and North Korea's increasing nuclear threat, Seoul's leadership is well aware that its most reliable security partner is the United States. As long as the US–ROK alliance remains strong, therefore, Seoul is likely to keep pursuing the deepening of the strategic partnership with Beijing, as it is necessary not only for South Korea's future economic prosperity but also for coping with North Korea's growing threats and provocations.