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Aquifer characteristics and water use data for 43 widely distributed small island states indicate that 44% are in a state of water stress. While recharge is projected to increase on 12 islands it is projected to decrease by up to 58% on the other 31. The majority of naturally occurring freshwater on small islands is groundwater, which is primarily recharged by precipitation1. Recharge rates are therefore likely to be impacted by climate change2. Freshwater resources on small islands are particularly vulnerable to climate change because they are limited in size and easily compromised3,4,5. Here we have compiled available aquifer system characteristics and water-use data for 43 small island developing states distributed worldwide, based on local expert knowledge, publications and regional data sets. Current vulnerability was assessed by evaluating the recharge volume per capita. For future vulnerability, climate change projections were used to estimate changes in aquifer recharge. We find that 44% of islands are in a state of water stress, and while recharge is projected to increase by as much as 117% on 12 islands situated in the western Pacific and Indian Ocean, recharge is projected to decrease by up to 58% on the remaining 31 islands. Of great concern is the lack of enacted groundwater protection legislation for many of the small island developing states identified as highly vulnerable to current and future conditions. Recharge indicators, shown alongside the state of legal groundwater protections, provide a global picture of groundwater supply vulnerability under current and future climate change conditions.