Water is a finite resource but an essential one. The continuously increasing demand leads to competition and conflict over limited resources. Syria, Jordan, and Israel compete over the water resources in the transboundary Yarmouk River basin where two water agreements dictate the allocation of water. The two arrangements are far from being efficient and fair while little cooperation is being made over sharing the resources that are being over-exploited. In this study, water sustainability was investigated under projected developments and trends based on the current use and allocation regime in the watershed using several scenarios. A one bucket soil moisture model was adopted and optimized in order to fully represent the ever-changing hydrology of the basin using the Water Evaluation and Planning (WEAP) tool. Demands along with the existing water infrastructure and their operation were modelled despite the lack of many data and the huge uncertainty in some. Simulation of business as usual scenario showed that continuing with the current use cannot be sustainable in the short and long term. The growth of both agriculture and population produced huge water shortages in all demand sectors. Under climate change scenarios RCP 4.5 and RCP 8.5, surface water availability and the retention of dams reduced significantly. The share of Jordan from the Yarmouk River was the most vulnerable to climate change impacts. Enhancing irrigation efficiency and a more stable population growth based on the UN medium variant population projection showed improvements in water coverage within all demand sectors. Analysis of future scenarios suggests that water shortage is expected in all riparian states of the basin but can be mitigated by reducing demands.