Deep rooting is an important factor affecting rice drought avoidance. However, few genes that control this trait have been identified in rice. In the present work, we cloned a gene, OsIAA8, associated with a rice quantitative trait locus for deep rooting. Overexpression of OsIAA8 increased deep rooting and yield under drought stress. OsIAA8, which encodes an Aux/IAA protein, interacts with auxin response factor OsARF12, and osarfl2 mutant plants also displayed increased deep rooting. The expression of several auxin transport genes (e.g., OsPIN2 and OsPIN8) was down-regulated in OsIAA8-overexpressing rice plants. OsARF12 promotes the transcription of these genes, whereas OsIAA8 inhibited this transcription activation. Furthermore, the auxin content in the roots of OsIAA8-overexpressing plants was reduced compared with wild type plants. Of five haplotypes at the OsIAA8 locus, accessions carrying Hap 2 had higher deep rooting. This study revealed that OsIAA8-OsARF12 module regulates deep rooting by inhibiting auxin transport, providing the insights into improvement of root architecture and drought resistance in rice breeding.