Grain size and grain weight are critical factors influencing crop yield. In rice ( L.), the auxin response factor (OsARF) family proteins, key components of the auxin signaling pathway, function as transcription factors and play essential roles in regulating various plant growth and development processes, including seed development. Here, we identified that () plays an essential role in regulating grain size and grain weight by activating the expression of () and (). The mutants showed larger grains with increased grain length, grain width, and 1000-grain weight. Furthermore, molecular evidence demonstrated that OsARF25 functions as a transcriptional activator. RNA-seq analysis further identified its target genes and . In addition, OsARF25 directly binds to the promoters of and and activates their expression. Further, the mutant exhibited enhanced sensitivity to brassinolide treatment, confirming that the targeting of and by OsARF25 mediates BR signaling. Taken together, our study revealed that functions as a regulator of grain length, grain width, and grain weight by participating in the BR signaling pathway, and it has potential value for molecular breeding in rice.