Virus-induced syncytium formation, or cell fusion, has been investigated as a potential therapeutic approach for cancer. HVJ virus infection or transfection of membrane glycoproteins of virions have been known to cause syncytium formation in affected cells. Also, it was known that other enveloped viruses such as Herpes viruses, leukaemia viruses induce cell fusion. However, substances or molecules that directly cause syncytium formation have not been identified to date. Here, we identify a peptide that efficiently induces syncytia and report its structure, as an actionable therapeutic agent for cancer and viral infection. We purified and identified the fusion factor from the exosomes of the cells infected with murine leukaemia virus but not producing viruses by column chromatography, mass spectrometry, and amino acid analyses. We confirmed the peptide purified from the cell culture media and synthesized peptides induce syncytia as well as the murine leukaemia viruses, or the membranes or exosomes of MuLV infected cell lines in RFL cells and several cancer cell lines leading to apoptosis. And this peptide suppresses in vivo growth of cancer cells significantly. Furthermore, we found the synthesized peptide can cause fusion of enveloped virions as well as virus infected cells or cancer cell lines. These results nominate the use of this peptide as a potentially promising therapeutic approach for cancer and viral infection through efficient induction of syncytium formation followed apoptosis.