The aragonitic micro-bivalve Carditella iejimensis, which is less than 3.5 mm in height and length, inhabits the sediment surface in a submarine cave off the Okinawa Islands, Japan. To evaluate the use of this species as a quantitative paleoceanographic proxy, we analyzed the δ¹⁸O values of 50 living C. iejimensis specimens collected from the Daidokutsu submarine cave (ca. 30 m water depth). Results show that most individuals (96%) preserve δ¹⁸O values corresponding to the mean annual temperature and δ¹⁸O of seawater. The mean δ¹⁸O_shell of the 50 specimens was -1.10 ± 0.18‰. These results, when applied to fossil shell δ¹⁸O records from Daidokutsu cave, indicate that its mean annual surface water temperature is about 1°C higher than temperatures during the Medieval Warm Period and the Middle Holocene Climatic Optimum, and that the recent warming is likely exceptional during the past 7000 years.