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Recent climate change has contributed to shifts in the seasonal interplay between precipitation and potential evapotranspiration, which have in turn increased droughts and reduced freshwater availability in Mediterranean climate regions. To overcome limitations in existing indices for comparing these seasonal hydroclimatic drivers at the global scale, we introduce an information theory‐based, nonparametric asynchronicity index that captures both the temporal alignment and relative magnitudes of precipitation and potential evapotranspiration. We use this asynchronicity index to first identify Mediterranean climates around the world. We then apply the asynchronicity index over two Mediterranean climate regions and show that their boundaries have shifted between 1960 and 2018, resulting in a regional expansion in the U.S. Pacific Northwest and a contraction in southwestern Australia. These results highlight the need for globally consistent measures of seasonal climatic water supply and demand for diagnosing potential changes in water resources and ecosystem responses within Mediterranean climate regions. Plain Language Summary: Climate change is likely to change the boundaries of what are typically considered Mediterranean climate regions, which have dry summers and mild, wet winters. To analyze how these regions' water availability may be influenced by climate change, we introduce a metric for quantifying the mismatch in the timing and amount of precipitation relative to atmospheric water demand. This new metric is able to identify the boundaries of Mediterranean climate regions more effectively than other existing indices and can be used to analyze how these boundaries shift over time. Key Points: A new metric is proposed to measure the seasonal mismatch between atmospheric water supply (precipitation) and demand (potential evapotranspiration, PET)Compared to existing metrics, the proposed metric can more accurately distinguish between climates with synchronized versus desynchronized seasonal signals of precipitation and PETMediterranean climates are characterized by high climate asynchronicity, and their geographical extents have expanded in the U.S. Pacific Northwest and contracted in Western Australia since the turn of the century [ABSTRACT FROM AUTHOR]