Mollusks are well known for their detailed recording of paleoenvironmental and paleoclimatic changes in their carbonate shells. In this study, we constructed 18-year blue color intensity and oxygen isotope profiles of a C-14 dated (AD 990 +/- 40) fossil giant clam, Tridacna gigas, from Shidao Island, South China Sea. The delta O-18 profile of the T. gigas specimen displayed regular annual cycles and was probably controlled by seasonal variations of the climatic parameters. The blue color intensity profile showed good agreement with the delta O-18 series, and both had 18 clear annual cycles in accordance with the 18 visually identified annual growth bands. The annual shell growth rate determined from the blue color intensity and oxygen isotope profiles indicated that the annual shell increment of the Tridacna specimen was stable after the onset of sexual maturity. Spectral analysis of the delta O-18 and blue color intensity time series suggested that the El Nino-Southern Oscillation (ENSO) period observed in the instrumental temperature and precipitation records of the South China Sea during the past 50 years also existed in medieval times. Our results showed that fossil giant clams could provide a good archive of historical intra-seasonal to decadal climate variations.