Secreted Semaphorin3A (Sema3A) proteins are known to act as diffusible and repellant axonal guidance cues during nervous system development. A receptor complex consisting of a Neuropilin and a Plexin-A mediates their effects. Plexin-A signal transduction has remained poorly defined despite the documented involvement of collapsin response mediator protein and molecule interacting with CasL proteins (MICALs) as mediators of Plexin-A activation. Here, we defined a domain of Plexin-A1 required for Sema3A signaling in a reconstituted environment and then searched for proteins interacting with this domain. RanBPM is shown to physically interact with Plexin-A1, and the RanBPM/Plexin complex is regulated by MICAL expression. Overexpression of RanBPM cooperates with PlexinA1 to reduce non-neuronal cell spreading and strongly inhibit axonal outgrowth in vitro and in vivo. A truncated RanBPM protein blocks Sema3A responsiveness in non-neuronal and neuronal cells. Suppression of RanBPM expression reduces Sema3A responsiveness. Thus, RanBPM is a mediator of Sema3A signaling through Plexin-A. RanBPM has the potential to link Plexin-A receptors to retrograde transport and microtubule function in axonal guidance.