Oncogenic RAS mutant (RAS) proteins have been considered undruggable via conventional antibody regimens owing to the intracellular location restricting conventional-antibody accessibility. Here, we report a pan-RAS-targeting IgG antibody, inRas37, which directly targets the intracellularly activated form of various RAS subtypes after tumor cell-specific internalization into the cytosol to block the interactions with effector proteins, thereby suppressing the downstream signaling. Systemic administration of inRas37 exerted a potent antitumor activity in a subset of RAS tumor xenografts in mice, but little efficacy in RAS tumors with concurrent downstream PI3K mutations, which were overcome by combination with a PI3K inhibitor. The YAP1 protein was up-regulated as an adaptive resistance-inducing response to inRas37 in RAS-dependent colorectal tumors; accordingly, a combination of inRas37 with a YAP1 inhibitor manifested synergistic antitumor effects in vitro and in vivo. Our study offers a promising pan-RAS-targeting antibody and the corresponding therapeutic strategy against RAS tumors.