Giant clams (genus Tridacna) are the largest living bivalves and, like reef-building corals, host symbiotic dinoflagellate algae (Symbiodinium) that significantly contribute to their energy budget. In turn, Symbiodinium rely on the host to supply inorganic carbon (Ci) for photosynthesis. In corals, host 'proton pump' vacuolar-type H+-ATPase (VHA) is part of a carbon-concentrating mechanism (CCM) that promotes Symbiodinium photosynthesis. Here, we report that VHA in the small giant clam (Tridacna maxima) similarly promotes Symbiodinium photosynthesis. VHA was abundantly expressed in the apical membrane of epithelial cells of T. maxima's siphonal mantle tubule system, which harbors Symbiodinium Furthermore, application of the highly specific pharmacological VHA inhibitors bafilomycin A1 and concanamycin A significantly reduced photosynthetic O2 production by ∼40%. Together with our observation that exposure to light increased holobiont aerobic metabolism ∼5-fold, and earlier estimates that translocated fixed carbon exceeds metabolic demand, we conclude that VHA activity in the siphonal mantle confers strong energetic benefits to the host clam through increased supply of Ci to algal symbionts and subsequent photosynthetic activity. The convergent role of VHA in promoting Symbiodinium photosynthesis in the giant clam siphonal mantle tubule system and coral symbiosome suggests that VHA-driven CCM is a common exaptation in marine photosymbioses that deserves further investigation in other taxa.