The Ras subfamily of small GTPases is mutated in ∼30% of human cancers and represents compelling yet challenging anticancer drug targets owing to their flat protein surface. We previously reported a bicyclic peptidyl inhibitor, cyclorasin B3, which binds selectively to Ras-GTP with modest affinity and blocks its interaction with downstream effector proteins in vitro but lacks cell permeability or biological activity. In this study, optimization of B3 yielded a potent pan-Ras inhibitor, cyclorasin B4-27, which binds selectively to the GTP-bound forms of wild-type and mutant Ras isoforms (KD = 21 nM for KRasG12V-GppNHp) and is highly cell-permeable and metabolically stable (serum t1/2 > 24 h). B4-27 inhibits Ras signaling in vitro and in vivo by blocking Ras from interacting with downstream effector proteins and induces apoptosis of Ras-mutant cancer cells. When administered systemically (i.v.), B4-27 suppressed tumor growth in two different mouse xenograft models at 1-5 mg/kg of daily doses.