Oncogenic RAS mutations drive cancers at many sites. Recent reports suggest that RAS dimerization, multimerization, and clustering correlate strongly with activation of RAS signaling. We have found that re-expression of DIRAS3, a RAS-related small GTPase tumor suppressor that is downregulated in multiple cancers, inhibits RAS/mitogen-activated protein kinase (MAPK) signaling by interacting directly with RAS-forming heteromers, disrupting RAS clustering, inhibiting Raf kinase activation, and inhibiting transformation and growth of cancer cells and xenografts. Disruption of K-RAS cluster formation requires the N terminus of DIRAS3 and interaction of both DIRAS3 and K-RAS with the plasma membrane. Interaction of DIRAS3 with both K-RAS and H-RAS suggests a strategy for inhibiting oncogenic RAS function. Display omitted •Re-expression of DIRAS3 (ARHI) inhibits ovarian, lung, and pancreatic cancer growth•DIRAS3 (ARHI) interacts directly with RAS at the plasma membrane•Re-expression of DIRAS3 inhibits RAS clustering and downstream MAPK signaling•The N-terminal extension of DIRAS3 is required for its tumor-suppressive function Sutton et. al. show that re-expression of DIRAS3 can inhibit the growth of multiple cancer types driven by K-RAS mutations by a direct interaction and disruption of K-RAS higher ordered clusters. This phenotype is driven by an N-terminal extension, which distinguishes DIRAS3 from other RAS-related small GTPases.