MYB-type transcription factors (MYB TFs) play diverse roles in plant development and stress responses. However, the mechanisms underlying the actions of MYB TFs during stress response remain unclear. In this study we identified a R2R3-MYB TF in soybean (Glycine max), denoted GmMYB84, which contributes to drought resistance. Expression of GmMYB84 was induced by drought, salt stress, H2O2 and ABA. Compared with the wild type (WT), GmMYB84-overexpressing soybean mutants (OE lines) exhibited enhanced drought resistance with a higher survival rate, longer primary root length, greater proline and reactive oxygen species (ROS) contents, higher antioxidant enzyme activities [peroxidase (POD), catalase (CAT) and superoxide dismutase (SOD)], a lower dehydration rate and reduced malondialdehyde (MDA) content. We also found that ROS could induce SOD/POD/CAT activity in OE lines. In particular, we found that the optimal level of ROS is required for GmMYB84 to modulate primary root elongation. Some ROS-related genes were up-regulated under abiotic stress in GmMYB84 transgenic plants compared with the WT. Furthermore, electrophoretic mobility shift assay and luciferase reporter analysis demonstrated that GmMYB84 binds directly to the promoter of GmRBOHB-1 and GmRBOHB-2 genes. Based on this evidence, we propose a model for how GmMYB84, H2O2 and antioxidant enzymes work together to control root growth under both optimal and drought stress conditions.