Ocean acidification poses a critical threat to marine calcifiers, yet the physiological mechanisms allowing certain species to thrive in high-CO₂ environments remain poorly understood. Kueishan Island (Taiwan) provides a unique natural laboratory to address this gap. While nearby non-vent reefs support diverse coral communities, the strongly acidified hydrothermal vent fields at Kueishan Island are dominated by the azooxanthellate coral <em>Tubastraea aurea</em>. Here, we tested whether this persistence is driven by regulation of the calcifying microenvironment. Using skeletal boron isotope (δ¹¹B) composition and scanning electron microscopy (SEM), we reconstructed calcifying-fluid pH (pH<sub>CF</sub>) and assessed skeletal microstructure in specimens from the vent site (HV, pH ~6.6) and a reference site (FU, pH ~8.0). Despite a ~1.4-unit difference in ambient seawater pH (pH<sub>SW</sub>), the estimated pH<sub>CF</sub> was indistinguishable between sites (HV 8.45 ± 0.02, FU 8.38 ± 0.01), resulting in a markedly larger ΔpH (pH<sub>CF</sub> − pH<sub>SW</sub>) at the vents. While SEM analysis revealed no gross dissolution at the resolution examined, vent colonies were smaller than reference counterparts, suggesting possible growth constraints under extreme conditions (exploratory given limited colony replication). These findings indicate that <em>T. aurea </em>maintains elevated pH<sub>CF</sub> in exceptionally low-pH seawater, highlighting a physiological capacity that could contribute to skeletal maintenance under future acidified oceans.