Grain size has long been recognized as a key determinant of yield potential in crops. Understanding the mechanisms governing grain size is critical for breeding high-yielding varieties. In a previous work, we revealed that the RNA-binding protein LARGE1 acts as a negative regulator of grain size and weight in rice. LARGE1 interacts with GSK2 (GLYCOGEN SYNTHASE KINASE2) and is phosphorylated by GSK2. Here, we report that LARGE1 physically interacts with an atypical non-DNA-binding bHLH protein PGL2 that positively influences grain size. Biochemical analyses show that PGL2 binds to APG, a typical DNA-binding bHLH protein that negatively regulates grain size. PGL2 suppresses the transcriptional activation activity of APG by forming the PGL2/APG heterodimer. Strikingly, LARGE1 can repress the formation of the heterodimer PGL2/APG by competitively binding PGL2, thereby releasing the inhibitory effect of PGL2 on the transcriptional activation activity of APG. Genetic evidence and RNA-seq analyses support that LARGE1 and PGL2 act in a common pathway to regulate grain size in rice. Our findings uncover a novel regulatory module GSK2-LARGE1-PGL2/APG that fine-tunes grain size, suggesting a promising target for improving seed size and weight in crops.