Variation in the stable isotopes of land-snail shells potentially provides useful information for reconstructing the terrestrial paleoenvironment. In this study, we investigated the relationship between vegetation and variation in the shell carbon (¹³C/¹²C) isotope ratios in the land snail Mandarina chichijimana on Chichijima of the Ogasawara Islands. In modern samples, the mean δ¹³C value of the adult shell (range -13.8∼-9.6) was higher at sites that contain a greater proportion of C₄ plants, especially near the coast (regression equation, mean δ¹³C=6.594×[C₄ plant proportion]-12.43). The shell δ¹³C of the living snails was also significantly positively correlated with the δ¹³C of the body tissues. As no differences were found in the δ¹³C values of shells collected in carbonate-rich areas and volcanic rock settings, we conclude that δ¹³C in M. chichijimana is almost exclusively influenced by the plants that they consume. Also, in fossil shells from limestone outcrops, the mean δ¹³C value (-11.6∼-11.4) was significantly lower than in modern shells from the same limestone outcrops (-11.0∼-9.6). This is therefore preliminary evidence that C₄ vegetation declined in line with a decrease in sea level around the time of the Last Glacial Maxima. Together, the findings may form a basis for the future use of land snail shells to estimate the paleoenvironment of the Pacific Islands in this region.