Abstract: Middle power identity and interests claimed by South Korean leaders predict a foreign policy of multilateralism, institution building, and contributions to global public goods. South Korea is indeed active in global governance, but its regional diplomacy for much of the Park Geun-hye administration defied middle power expectations. In recent years, Seoul appeared to apply a strategy of isolating and pressuring Tokyo, while behaving like a smaller power showing deference to Beijing. Existing literature offers several explanations for failures to implement middle power diplomacy: historical memory impediments (e.g., Japan), budgetary constraints (e.g., Canada and Australia), stalled regionalization (Brazil and Turkey), and inadequate economic development (India and Indonesia). Finding these explanations insufficient for the South Korean case, this article shows how anti-Japan identity and Korean unification interests at times overwhelmed South Korean middle power identity and interests, respectively. The article offers implications for the growing category of states considered middle powers and concludes with policy recommendations for how Seoul can adjust its mismatched diplomacy to serve as a constructive middle power in Asia.