"Cell-in-cell" denotes an invasive phenotype in which one cell actively internalizes in another. The novel human T-cell line HOZOT, established from human umbilical cord blood, was shown to penetrate a variety of human cancer cells but not normal cells. Oncolytic viruses are emerging as biological therapies for human cancers; however, efficient viral delivery is limited by a lack of tumor-specific homing and presence of pre-existing or therapy-induced neutralizing antibodies. Here, we report a new, intriguing approach using HOZOT cells to transmit biologics such as oncolytic viruses into human cancer cells by cell-in-cell invasion. HOZOT cells were successfully loaded via human CD46 antigen with an attenuated adenovirus containing the fiber protein of adenovirus serotype 35 (OBP-401/F35), in which the telomerase promoter regulates viral replication. OBP-401/F35-loaded HOZOT cells were efficiently internalized into human cancer cells and exhibited tumor-specific killing by release of viruses, even in the presence of anti-viral neutralizing antibodies. Moreover, intraperitoneal administration of HOZOT cells loaded with OBP-401/F35 significantly suppressed peritoneally disseminated tumor growth in mice. This unique cell-in-cell property provides a platform for selective delivery of biologics into human cancer cells, which has important implications for the treatment of human cancers.