The dispute over the Diaoyu/Senkaku Islands is generally analysed as a Sino-Japanese competition over material and strategic interests, regional preponderance, and nationalistic symbolism. Yet, such explanations cannot fully explain the endurance of the conflict and overlook its origin in the period leading up to the UN’s derecognition of Taiwan’s sovereignty in 1971. Drawing on the concept of ontological security, defined as ‘security of the self’, we contend that it was the looming loss of its sovereign self that prompted Taiwan (Republic of China, ROC) to assert itself as the true defender of Chinese interests by laying claim to the islands. This caused anxiety in China (People’s Republic of China, PRC), which had to follow suit in order to secure its own sovereign self. China thus inherited the conflict with Japan when it took over the ‘true China’ mantle upon its entry to the UN in 1971. Extant explanations overlook the important factor of inter-Chinese competition over sovereign selfhood. In developing this argument, the article makes two contributions. First, it draws attention to a much-overlooked early phase of the dispute, and shows how the same dynamics of ROC–PRC status competition continue to inform the dispute between China and Japan today. Second, it contributes to the literature on ontological security by conceptualising the ‘self’ as sovereign state personhood, thereby further clarifying the distinction between self and identity, and highlighting the relational effects of ontological security-seeking.