Regulation of seed size is a key strategy for improving crop yield, and is also a basic biological question, but how plants determine their seed size remains elusive. Here we report that the GW2-WG1-OsbZIP47 regulatory module controls grain width and weight in rice. WG1, which encodes a glutaredoxin protein, promotes grain growth by increasing cell proliferation. WG1 interacts with the transcription factor OsbZIP47 and represses its transcriptional activity by associating with the transcriptional co-repressor ASP1, indicating that WG1 may act as adaptor protein to recruit the transcriptional co-repressor. OsbZIP47 restricts grain growth by decreasing cell proliferation. Further results reveal that the E3 ubiquitin ligase GW2 ubiquitinates WG1 and targets it for degradation. Genetic analyses support that GW2, WG1 and OsbZIP47 function in a common pathway to control grain growth. Thus, our findings reveal a genetic and molecular framework for the GW2-WG1-OsbZIP47 regulatory module-mediated control of grain size and weight, opening new perspectives for using this regulatory pathway for improvement of seed size and weight in crops.