This study presents a comprehensive compilation of high-resolution, continuous time series of upper ocean pH data collected using autonomous sensors across various marine ecosystems, ranging from polar to tropical, open-ocean to coastal, kelp forest to coral reef. The data reveal a wide range of month-long pH variability, with standard deviations from 0.004 to 0.277 and ranges spanning 0.024 to 1.430 pH units. The observed variability is highly site-dependent, with characteristic diel, semi-diurnal, and stochastic patterns of varying amplitudes. These biome-specific pH signatures indicate that resident organisms are already exposed to pH regimes that are not predicted to occur until 2100. The study highlights the importance of understanding spatial and temporal variation in seawater chemistry for improving the design of Ocean Acidification (OA) experiments and for guiding management efforts to identify areas of marine habitat that can serve as refugia to acidification and areas that are particularly vulnerable to future ocean change. The data provide a crucial step in linking environmental history of pH exposure to the physiological resilience of marine organisms to fluctuations in seawater CO₂.This study presents a comprehensive compilation of high-resolution, continuous time series of upper ocean pH data collected using autonomous sensors across various marine ecosystems, ranging from polar to tropical, open-ocean to coastal, kelp forest to coral reef. The data reveal a wide range of month-long pH variability, with standard deviations from 0.004 to 0.277 and ranges spanning 0.024 to 1.430 pH units. The observed variability is highly site-dependent, with characteristic diel, semi-diurnal, and stochastic patterns of varying amplitudes. These biome-specific pH signatures indicate that resident organisms are already exposed to pH regimes that are not predicted to occur until 2100. The study highlights the importance of understanding spatial and temporal variation in seawater chemistry for improving the design of Ocean Acidification (OA) experiments and for guiding management efforts to identify areas of marine habitat that can serve as refugia to acidification and areas that are particularly vulnerable to future ocean change. The data provide a crucial step in linking environmental history of pH exposure to the physiological resilience of marine organisms to fluctuations in seawater CO₂.