Background: Synovial sarcoma is one of malignant soft tissue tumors with a poor prognosis. A cancer-testis antigen NY-ESO-1 is expressed in the tumors of 50-80% synovial sarcoma patients. The purpose of this study was to evaluate the safety and the efficacy of gene engineered autologous T cell product with NY-ESO-1 siTCR retroviral vector which expressed affinity-enhanced NY-ESO-1-specific TCR and siRNA to silence endogenous TCR (TBI-1301). Methods: This was an open label phase I/II study to evaluate safety, appearance of replication competent retrovirus (RCR), appearance of clonality, in vivo cell kinetics and clinical responses. TBI-1301 were manufactured from each subject’s leucocytes stimulated with anti-CD3 antibody and Retronectin and transduced with NY-ESO-1 siTCR retroviral vector. TBI-1301 was infused at split dose of 5 x 10⁹ cells following cyclophosphamide treatment at 750 mg/m² for two days to HLA-A*02:01 or HLA-A*02:06 positive subjects with synovial sarcoma expressing NY-ESO-1, which were surgically unresectable and refractory to anthracycline therapy. Results: Eight subjects were enrolled and treated with TBI-1301. The objective response rate was 50.0% with best overall partial response in 4 of 8 subjects according to RECIST v1.1/irRECIST and the median overall survival was 650 days. Cytokine release syndrome (CRS) occurred in 50.0% (4/8) and consisted of 1 subject with grade 1 CRS and 3 subjects with grade 2 CRS. All subjects who developed CRS recovered with prespecified treatment, in which 2 subjects were treated with symptomatic therapy, 1 subject was treated with tocilizumab and 1 subject was treated with both tocilizumab and corticosteroid. No subjects had immune effector cell- associated neurotoxicity syndrome (ICANS). RCR and clonal dominance were not detected in any subjects throughout the study period. Conclusions: Adoptive immunotherapy with TBI-1301 to selectively target NY-ESO-1 positive tumors will become a promising treatment for advanced or recurrent synovial sarcoma with acceptable toxicity. Clinical trial information: NCT03250325.