Background For the diagnosis and treatment of adult T‐cell leukemia/lymphoma (ATLL) caused by human T‐lymphotropic virus type 1 (HTLV‐1) are required therapeutic modalities urgently. Non‐human primate models for ATLL would provide a valuable information for clinical studies. We did a pilot study to establish an ATLL non‐human primate model using common marmosets (Callithrix jacchus). Methods We inoculated HTLV‐1–producing MT‐2 cells into 9‐month‐old marmosets, either intraperitoneally or intravenously. We next administrated MT‐2 cells into 13‐month‐old marmosets under cyclosporine A (CsA) treatment to promote infection. HTLV‐1 infection was determined by measuring HTLV‐1 antibody titer in the common marmosets. Results The HTLV‐1 antibody titer increased in the intraperitoneally inoculated marmoset with or without CsA treatment, and it kept over five 5 years though proviral copy number (proviral load, PVL) remained low throughout the study. Conclusion We obtained HTLV‐1 asymptomatic carriers of common marmosets by inoculating MT‐2 cells.