The regulation of intracellular pH (pHi) is a fundamental aspect of cell physiology that has received little attention in studies of reef-building corals and symbiotic cnidarians. Here, we investigated the hypothesis that dynamic changes in the pHi of coral host cells are controlled by the photosynthetic activity of the coral's dinoflagellate symbionts. Using live cell imaging and the pH-sensitive dye SNARF-1, we tracked pH in symbiont-containing and symbiont-free cells isolated from the reef coral Stylophora pistillata. We characterised the response of coral pHi in the presence of a photosynthetic inhibitor, the dynamics of coral pHi during light exposure and how pHi values vary on exposure to a range of irradiance levels lying within the coral's photosynthesis-irradiance response curve. Our results demonstrate that increases in coral pHi are dependent on photosynthetic activity of intracellular symbionts and that pHi recovers under darkness to values that match those of symbiont-free cells. Furthermore, we show that the timing of the pHi response is governed by irradiance level and that pHi increases to irradiance-specific steady-state values. Minimum steady-state pHi values of 7.05 ± 0.05 were obtained under darkness and maximum values of 7.46 ± 0.07 were obtained under saturating irradiance. As changes in pHi often affect organism homeostasis, there is a need for continued research into acid/base regulation in symbiotic corals. More generally, these results represent the first characterization of photosynthesis-driven pHi changes in animal cells.