Optimizing rice (Oryza sativa L.) plant architecture and seed germination is critical for improving agronomic performance. Here, we investigated the role of OsARF9, a class B auxin response factor, in regulating these traits. Through analysis of OsARF9 transgenic lines, combined with phenotypic characterization, hormone response assays, and promoter binding studies, we demonstrate that OsARF9 functions as a transcriptional repressor. OsARF9 transgenic lines display clear differences in architecture traits and genotype-dependent responses to auxin, brassinosteroid (BR), and abscisic acid (ABA) treatments. OsARF9 directly binds promoter elements beyond the canonical auxin response element (AuxRE) and represses expression of OsGH3-5 and OsRLA1. Furthermore, it binds the promoters of OsABI3 and OsEm1, reduces their transcription, and thereby modulates ABA-mediated inhibition of seed germination. Collectively, our results establish OsARF9 as an integrative transcriptional repressor that coordinates auxin, BR, and ABA signaling to shape rice architecture and seed germination.