Mutations in KRAS are detected in various human cancers. However, there are few effective drugs for KRAS-mutated cancers. Glutamine is thought to be one of the pivotal nutrients for proliferation and survival of cancer cells. ASCT2 (SLC1A5) is a multi-pass transmembrane protein that transports neutral amino acids. ASCT2 is reported to be highly expressed in many cancer cells, and is thought to work as a major transporter for glutamine. We have developed a novel monoclonal antibody (mAb) against the extracellular domain of human ASCT2. In the present study, we examined the effects of the mAb on KRAS-mutated colorectal cancer cells. Treatment with anti-ASCT2 mAb decreased the concentration of intracellular glutamine and in vivo tumor growth. However, these effects were not observed in colorectal cancer cells which harbor wild-type KRAS. These results suggest that ASCT2 is a promising therapeutic target for KRAS-mutated cancers.