Whereas the GTPase RhoA has been shown to promote proliferation and malignant transformation, the involvement of RhoB in these processes is not well understood. In this manuscript RhoB is shown to be a potent suppressor of transformation and human tumor growth in nude mice. In several human cancer cell lines, RhoA promotes focus formation whereas RhoB is as potent as the tumor suppressor p53 at inhibiting transformation in this assay. RhoB is both farnesylated (F) and geranylgeranylated (GG), and RhoB-F has been suggested as a target for the antitumor activity of farnesyltransferase inhibitors. Here we demonstrate that both RhoB-F and RhoB-GG inhibit anchorage-dependent and -independent growth, induce apoptosis, inhibit constitutive activation of Erk and insulin-like growth factor-1 stimulation of Akt, and suppress tumor growth in nude mice. The data demonstrate that RhoB is a potent suppressor of human tumor growth and that RhoB-F is not a target for farnesyltransferase inhibitors.