The world’s major river deltas provide bountiful ecosystem services to hundreds of millions of people1,2, but there are widespread concerns that their ability to support these critical functions is being threatened by increases in salinity and the extent of tidal zones3, forced by sea-level rise4. Changes in the position of the fluvial-marine interface are governed not only by climate-driven sea-level rise, but by other factors including variations in river discharge and channel geometry5,6. However, the relative contributions of different driving factors to shifting tidal extent remains unconstrained. Here we use new field data and hydrodynamic modelling to quantify the influence of these different driving factors on accelerating tidal extension in the Mekong delta, both for the recent past and projected into the future under a range of policy, regulation and climate change scenarios. Our results indicate that, within the next two decades, tidal extension into the delta will increase by up to 56 km due to channel deepening, primarily driven by anthropogenic sediment starvation, which accounts for almost all (92%) of this incursion. Eustatically rising sea-levels are found to contribute only modestly. These findings will be transferable to other large sediment starved deltas that are facing similar challenges.