Grain size is an important agronomic trait and influences both grain yield and quality in crops. The atypical heterotrimeric Gγ protein subunit GS3 is a central regulator of grain length in rice, and the loss-of-function allele of its corresponding gene has been widely utilized by breeders to improve grain length in rice. Here we report that the CC-type glutaredoxin WG1/OsGRX8 has disulfide oxidoreductase activity and regulates redox state of GS3, thereby determining grain length in rice. GS3 can form dimers and oligomers by intermolecular disulfide bonds, and the cysteine-rich C-terminal region of GS3 is predominantly required for its oligomerization. The oligomerization of GS3 alleviates its inhibitory effect on the interaction between RGB1 and DEP1/GGC2, resulting in an increase in grain length. WG1 interacts with GS3 and reduces the oligomerization of GS3 through redox mechanisms, which causes a decrease in grain length. Genetic analyses support WG1 and GS3 function in a common pathway to control grain length. Thus, our findings reveal a previously unrecognized mechanism, in which redox regulation of a Gγ subunit by a glutaredoxin controls grain length, opening a novel perspective for G protein signaling regulation.