Introduction: Next‐generation sequencing (NGS)‐based circulating tumor DNA (ctDNA) detection is a promising monitoring tool for lymphoid malignancies. Studies for T cell lymphoma are limited. Methods: We explored whether this technology is applicable to T cell lymphoma with different subtypes and assessed its performance in clinical settings. Results: Thirty tumor and 74 blood samples were analyzed in our study. Malignant clone was identified in 23 of the 30 (76.7%) tumor samples through high‐throughput sequencing (HTS) combined with PCR. We detected the same tumor clone in plasma in 18out of the 23 (78.3%) patients. Circulating tumor DNA fraction correlated with lactate dehydrogenase (LDH) (r =.52, P =.017), high level of ctDNA predicted treatment failure (P =.0003) and there was a trend patients with high ctDNA burden would have poorer PFS Furthermore, ctDNA changed in concordance with clinical outcome and was more sensitive than PET/CT. Also, recurrence of ctDNA was an important clue for relapse. Conclusion: In conclusion, our study indicated that ctDNA monitoring was suitable for T cell lymphoma. High level of pretreatment ctDNA was a poor prognosis factor and changes of ctDNA correlated well with clinical courses and was sensitive to find early relapse. [ABSTRACT FROM AUTHOR]