South Asia is home to some of the world’s most critical transboundary river systems—the Indus, Ganges, and Brahmaputra—which are vital for the region’s agriculture-dependent economies and rapidly growing populations. However, water sharing in this region remains deeply contested, often constrained by historical grievances, political mistrust, and fragmented bilateral relations. This paper explores the complex interlinkages between transboundary water management, regional security, and geopolitical rivalry in South Asia, with a focus on India’s central role and the increasing influence of China. Water security has emerged as a sensitive and strategic concern, with states prioritising national development and sovereignty over regional cooperation. Existing water sharing treaties—such as the Indus Waters Treaty (1960) and the Ganges Treaty (1996)—are examined in terms of effectiveness, trust deficit, and asymmetry in power perceptions. India, often seen as a “big brother” with hegemonic intent, is viewed with suspicion by its neighbours, further complicating water diplomacy. The paper argues that the growing Chinese presence, particularly via the Belt and Road Initiative and upstream control of major rivers, is altering regional equations. As climate change intensifies water scarcity, the likelihood of internal instability and cross-border tensions will rise. Hence, a regional cooperative mechanism is urgently needed, one that addresses not only equitable water distribution but also long-term security and sustainability. As the largest regional power, India must lead trust-building efforts and support transparent, inclusive mechanisms under regional forums like SAARC. While diplomacy must remain the first line of action, the paper acknowledges that India may, in worst-case scenarios, need to assert its strategic leverage to safeguard its interests. Ultimately, managing South Asia’s transboundary waters equitably and cooperatively is essential not just for national water security, but for lasting regional peace and resilience against external geopolitical pressures.