We report carbon (δ¹³C) and oxygen (δ¹⁸O) isotope records of two modern giant clam (Tridacna squamosa) shells from two sites (Ishigaki-jima and Okinoerabu-jima) at different latitudes in the Ryukyu Islands, Japan. The δ¹³C profiles of samples from the inner shell layer on cross-sections along the maximum growth axis display no ontogenetic trends or seasonal variations. This finding suggests that the calcification site is likely to be unaffected by CO₂ uptake and release resulting from the metabolic activity of the molluscan host and algal symbionts. The δ¹⁸O profiles show distinct seasonal cycles. After accounting for the influence of seawater δ¹⁸O, the time-series variations are consistent with variations in sea surface temperature, and the temperature dependency of oxygen isotope fractionation is nearly identical to previously published δ¹⁸O–temperature relationships for biogenic and synthetic aragonite. We conclude that δ¹⁸O records from pristine fossils of this species will enable accurate paleoenvironmental reconstructions at high temporal resolution.