SAH-SOS1A TFA is a peptide-based SOS1/KRAS protein interaction inhibitor. SAH-SOS1A TFA binds to wild-type and mutant KRAS (G12D, G12V, G12C, G12S, and Q61H) with nanomolar affinity (EC50=106-175 nM). SAH-SOS1A TFA directly and independently blocks nucleotide association. SAH-SOS1A TFA impairs KRAS-driven cancer cell viability and exerts its effects by on-mechanism blockade of the ERK-MAPK phosphosignaling cascade downstream of KRAS.