Rnd proteins are atypical members of the Rho GTPase family that induce actin cytoskeletal reorganization and cell rounding. Rnd proteins have been reported to bind to the intracellular domain of several Plexin receptors, but whether Plexins contribute to the Rnd-induced rounding response is not known. Here we show that Rnd3 interacts preferentially with Plexin-B2 of the three Plexin-B proteins, whereas Rnd2 interacts with all three B-type Plexins, and Rnd1 shows only very weak interaction with Plexin-B proteins in immunoprecipitations. Plexin-B1 has been reported to act as a GAP for R-Ras and/or Rap1 proteins. We show that all three Plexin-B proteins interact with R-Ras and Rap1, but Rnd proteins do not alter this interaction or R-Ras or Rap1 activity. We demonstrate that Plexin-B2 promotes Rnd3-induced cell rounding and loss of stress fibres, and enhances the inhibition of HeLa cell invasion by Rnd3. We identify amino acids in Rnd3 required for Plexin-B2 interaction, and show that mutation of these amino acids prevents Rnd3-induced morphological changes. These results indicate that Plexin-B2 is a downstream target for Rnd3 that contributes to its cellular function.