An increase in grain yield is crucial for modern agriculture¹. Grain size is one of the key components of grain yield in rice and is regulated by quantitative trait loci (QTLs)^2,3. Exploring new QTLs for grain size will help breeders develop elite rice varieties with higher yields^3,4. Here, we report a new semi-dominant QTL for grain size and weight (GS2) in rice, which encodes the transcription factor OsGRF4 (GROWTH-REGULATING FACTOR 4) and is regulated by OsmiR396. We demonstrate that a 2？bp substitution mutation in GS2 perturbs OsmiR396-directed regulation of GS2, resulting in large and heavy grains and increased grain yield. Further results reveal that GS2 interacts with the transcription coactivitors OsGIF1/2/3, and overexpression of OsGIF1 increases grain size and weight. Thus, our findings define the regulatory mechanism of GS2, OsGIFs and OsmiR396 in grain size and weight control, suggesting this pathway could be used to increase yields in crops.