Grain size is one of the important yield traits in crops. Understanding the molecular and genetic mechanisms of grain-size control is important for yield improvement. Here, we report that the enhancer of GS2AA (EOG1) encodes an RNA-binding protein, which can bind mRNAs of several grain-size genes and influence their abundance. The eog1-1 mutant produces large and heavy grains by promoting cell proliferation in the spikelet hull. OsGSK3 physically interacts with and phosphorylates EOG1, thereby influencing the stability of EOG1. Genetic analyses support that EOG1 and OsGSK3 share overlapped function in grain size and weight control but does so independently of GS2. Notably, genome editing of wheat homologs TaEOG1A/B/D causes large and heavy grains. Thus, our findings identify a genetic and molecular mechanism whereby the OsGSK3-EOG1 module regulates grain size and weight in rice, suggesting that this pathway has the potential for grain-size improvement in key crops.