Tridacna shells are valuable archives in paleoclimate research due to their incremental growth bands, which record environmental conditions such as sea surface temperature (SST) and salinity through variations in oxygen isotopes (δ18O). Their potential for reconstructing past climate variability makes them a critical tool for understanding El Niño Southern Oscillation (ENSO), the primary source of interannual climate variability on Earth. In this study, we validate the use of two understudied species of Tridacna, Tridacna derasa and Hippopus hippopus, from Palau to identify ENSO signatures through oxygen isotopic analysis. Stable oxygen isotope ratios from three specimens were analyzed, and environmental signatures across species and locations (Malakal Harbor and West Passage) were compared. The Extended Reconstructed Sea Surface Temperature (ERSST) dataset from NOAA, the Multivariate ENSO Index extended version dataset (MEI.ext) from NOAA, and a Palauan coral δ18O record from Osborne et al. 2014 were used as baseline ENSO comparisons. The results indicate that both species of Tridacna record ENSO-related climate signals, with the younger H. hippopus from West Passage exhibiting the most pronounced δ18O fluctuations—particularly during La Niña events—suggesting a stronger response to precipitation-driven freshwater input. Conversely, the two shells from Malakal Harbor showed more muted isotopic variations but clearer ENSO signals, potentially reflecting site-specific variations due to differences in local hydrology and minimal exposure to freshwater runoff. These findings contribute to improving paleoclimate reconstructions using Tridacna shells by validating both species’ use for further ENSO research and highlighting the importance of site selection in isotopic studies. Future research is needed to investigate older Tridacna specimens to extend ENSO records further back in time and analyze additional environmental proxies, such as trace element ratios, to refine climate reconstructions.