This article critically evaluates the basis, aspirations, and prospects for bilateral security cooperation between Japan and South Korea. The assumption that common enemies, friends, and interests should have given rise to solid and steadily improving relations between Tokyo and Seoul has been far from realized. Rather, the Japan-ROK relationship continues to be marked by highly volatile behavior – ranging from intense friction to reluctant cooperation – which not only offers a vexing puzzle to the Realist school of international relations but also to the Constructivist one. This article argues that despite the perceived improvement in relations, Japan-South Korea security cooperation is situational and limited; recent developments do not imply a fundamental realignment of the two powers towards one another.