Despite extensive studies on the effects of elevated atmospheric CO₂ concentrations ([CO₂]) on rice, the molecular mechanisms and signaling events underlying the adaptation of plants remain largely elusive. Here, we report that OsRab6a, which encodes a small GTPase, is involved in the regulation of rice growth, grain yield, and accumulation of iron (Fe) in response to elevated [CO₂] (e[CO₂]). We generated transgenic plants with OsRab6a-overexpression (-OE) together with OsRab6a-RNAi lines, and found no differences in growth and grain yield among them and wild-type (WT) plants under ambient [CO₂] conditions. Under e[CO₂] conditions, growth and grain yield of the WT and OsRab6a-OE plants were enhanced, with a greater effect being observed in the latter. In contrast, there were no effects of e[CO₂] on growth and grain yield of the OsRab6a-RNAi plants. Photosynthetic rates in both the WT and OsRab6a-OE plants were stimulated by e[CO₂], with the magnitude of the increase being higher in OsRab6a-OE plants. Fe concentrations in vegetative tissues and the grain of the WT and transgenic plants were reduced by e[CO₂], and the magnitude of the decrease was lower in the OE plants than in the WT and RNAi plants. Genes associated with Fe acquisition in the OsRab6a-OE lines exhibited higher levels of expression than those in the WT and the RNAi lines under e[CO₂]. Analysis of our data using Dunnett’s multiple comparison test suggested that OsRab6a is an important molecular regulator that underlies the adaptation of rice to e[CO₂] by controlling photosynthesis and Fe accumulation.