This article explores China’s evolving approach to South–South security cooperation, focusing on its use of the security–development nexus in its relations with Global South countries, particularly in the Mekong region. The central research question is: under what circumstances will China adjust the balance between security and development in its cooperation with the Global South? This article uses two case studies in the Mekong region, where China has applied contrasting approaches to address different security-related issues, to explore how China justifies its security choices through the security–development nexus in its relations with Global South countries. The varying security priorities and complex regional dynamics have led to variations in the nature and scope of its security cooperation with the Global South. Drawing on Chinese official documents, media reports, and interviews with scholars and policy advisors, this article argues that China adapts its security–development approach based on three key factors: its perception of security risks, its ability to dominate an issue, and the urgency of its interests. The significance of this research lies in its contribution to the ongoing debate about China’s role in South–South security cooperation. The study also offers insights into the challenges China faces in managing its dual role as both a development partner and a security actor, with implications for its influence in the Mekong region and beyond.