Many regions in Chile experienced an unprecedented drought from 2010 to 2022, driven by climate change and natural variability. This so-called megadrought led to severe water scarcity, causing conflicts and exposing issues in Chilean water regulations. Water-intensive agriculture in areas with limited water availability has worsened these problems, raising questions about the contributions of water extraction and climate on high water stress levels.</p> <p>In this study, we evaluate water stress in Chile over the long term, from the mid-20th century to the end of the 21st century, under various climate and socio-economic scenarios. To this end, novel datasets of water availability, land use and water use were developed. Using these, we calculated the Water Stress Index (WSI) for all major basins in the country and assessed the impact of increasing water use and climate change on water stress over different time periods. Results show that most basins in semi-arid regions experienced high to extreme water stress (WSI > 40 % and WSI > 70 %, respectively) during the megadrought, mainly due to reduced water availability, but worsened by high water demand. Over time, increasing water stress in central Chile is primarily linked to rising water consumption, with a smaller contribution from water availability changes, leading to consistently high water stress levels (1990–2020 average) in several basins from Santiago northward. Under an adverse climate scenario (SSP3-7.0), megadrought-like conditions could become permanent by the end of the 21st century, with a projected 30 % drop in precipitation, resulting in high to extreme water stress in most basins in central Chile. We argue that using the WSI to assess one of the several aspects of water security offers a valuable strategy for adaptation plans. If public policy agrees on establishing quantifiable water security goals based on metrics like the WSI, different pathways of water use combined with alternative water sources can be evaluated to achieve them.