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Ongoing ocean acidification is known to be a major threat to tropical coral reefs. To date, only few studies have evaluated the impacts of natural long-term exposure to low-pH seawater on the chemical regulation and growth of reef-building corals. This work investigated the different responses of the massive Porites coral living at normal (pH sw  ~ 8.03) and naturally low-pH (pH sw  ~ 7.85) seawater conditions at Palau over the last decades. Our results show that both Porites colonies maintained similar carbonate properties (pH cf , [CO 3 2- ] cf , DIC cf, and Ω cf ) within their calcifying fluid since 1972. However, the Porites skeleton of the more acidified conditions revealed a significantly lower density (~ 1.21 ± 0.09 g·cm -3 ) than the skeleton from the open-ocean site (~ 1.41 ± 0.07 g·cm -3 ). Overall, both Porites colonies exerted a strong biological control to maintain stable calcifying fluid carbonate chemistry that favored the calcification process, especially under low-pH conditions. However, the decline in skeletal density observed at low pH provides critical insights into Porites vulnerability to future global change., Competing Interests: Declaration of competing interest The authors declare that they have no known competing financial interests or personal relationships that could have appeared to influence the work reported in this paper., (Copyright © 2023 Elsevier B.V. All rights reserved.)