Respiratory burst oxidase homologs (RBOHs) constitute a multigene family in plants. These reactive oxygen species (ROS)-generating enzymes participate in diverse biological processes. We previously demonstrated that PvRbohB plays an important role in lateral root (LR) development in Phaseolus vulgaris. However, little is known about the roles of other Rboh members in root development. Here, we report that PvRbohA is involved in LR emergence in P. vulgaris. PvRbohA was expressed in wild-type P. vulgaris root tissues, e.g., the radicle, inter LR zone, and LR zone, and its expression increased during LR formation. Analysis of the spatio-temporal expression patterns of a reporter construct under the control of the PvRbohA promoter (PvRbohA::GUS-GFP) in transgenic roots showed that PvRbohA was active at all three stages of LR development, but its spatial expression pattern varied at each stage. The relative expression levels of PvRbohA during LR formation correlated with the activity of PvRbohA promoter. Furthermore, upon PvRbohA transcript silencing, LR growth was significantly altered in transgenic hairy roots. These findings suggest that RbohA participates in LR initiation, emergence, and development in the legume P. vulgaris by delimiting the region for LR emergence.