<p>Volcanic gases erupt from the seafloor in several regions around Japan. Volcanological and geochemical gas seep studies have mainly focused on coastal shallow-water areas that are relatively accessible and important to human society. Shallow-water CO₂ seeps are thought to foreshadow future marine environments that may develop if CO₂ emissions are not drastically reduced. Thus, CO₂ seeps provide important insights for assessing and projecting the impacts of ocean acidification on marine ecosystems. This study is the first to investigate two shallow-water CO₂ seeps near Japan from the perspective of ocean acidification. We observed biotic transitions and reduced biodiversity around these CO₂ seeps, as well as high CO₂ concentrations, low pH, and low calcium carbonate saturation—conditions expected to occur by the end of this century unless anthropogenic CO₂ emissions are significantly reduced. These results suggest that, from a marine life conservation perspective, it is essential to mitigate ocean acidification through substantial reductions in anthropogenic CO₂. Shallow-water CO₂ seeps serve as natural experimental sites that illustrate ocean acidification and its effects on marine ecosystems. Given that the shallow-water CO₂ seeps examined in this study are both located in geoparks, study tours and ecotourism field trips should utilize these sites to enhance awareness of the consequences of ocean acidification and climate change.</p><p></p>