L-type amino acid transporter 1 (LAT1/SLC7A5), which transports large neutral amino acids, is highly upregulated in various cancer cells and supports their enhanced growth and proliferation. Previous immunohistochemical studies have reported significant correlations between the high LAT1 expression and the poor prognosis of patients in multiple cancer types. Furthermore, several LAT1-selective inhibitors are currently under evaluation as novel anti-cancer drugs. Detecting the LAT1 expression levels in tumor lesions with minimally invasive methods, therefore, would be beneficial for prognosis and diagnosis of cancers, including the companion diagnosis in the upcoming LAT1-targeted therapy. Molecules on exosomes, small extracellular vesicles, released from cancer cells are emerging as a novel class of biomarkers. In this study, we found that LAT1 is detectable in exosomes isolated from the cell culture supernatants of cancer cells. Notably, the abundance of LAT1 on exosomes was associated with its expression level in the cancer cells from which they derived. LAT1 was also present in exosomes isolated in vivo from peritoneal washing fluid of intraperitoneal tumor-bearing mice. These results indicate that LAT1 on cancer cell-derived exosomes holds a significant potential as a biomarker for diagnosis and prognosis of cancers.