Introduction: The clinical implications of plasma Epstein-Barr virus (EBV) DNA in patients with peripheral T-cell lymphoma (PTCL) remain unclear. Objective: This study aimed to explore the clinical correlations of pre- and post-treatment plasma EBV-DNA concentrations with treatment outcomes and prognosis in patients with PTCL. Methods: We retrospectively reviewed 128 patients diagnosed with PTCL with available data on pre-treatment plasma EBV-DNA, including 63 patients for whom post-treatment plasma EBV-DNA data were also available. Patients with extra-nodal NK/T-<X00_Del_TrennDivis></X00_Del_TrennDivis>cell lymphoma were excluded from this study. Results: Pre-treatment plasma EBV-DNA was elevated (e.g., ≥500 copies/mL) in 35.2% of PTCL patients, with significant differences in positive rates between different subtypes of PTCL (p < 0.001). High pre-treatment EBV-DNA concentrations were associated with advanced age (>60 years), elevated lactate dehydrogenase levels, high International Prognostic Index (IPI), and positive EBV-encoded RNAs-ISH in tumor specimens. In multivariate analyses, pre-treatment EBV-DNA ≥500 copies/mL was an independent prognostic factor after adjusting for IPI and pathological subtypes (hazard ratio = 2.14, p = 0.032). For patients with elevated pre-treatment EBV-DNA, normalization of EBV-DNA concentrations after first-line chemotherapy was significantly associated with better overall response rate (81.3% vs. 22.2%, p = 0.014), progression-free survival (12.0 months vs. 3.7 months, p = 0.011), and overall survival (37.9 months vs. 7.8 months, p = 0.012). For patients achieving remission to first-line therapy, rebound of EBV-DNA levels during follow-up was associated with disease relapse or progression. Conclusions: These results suggest that pre-treatment plasma EBV-DNA concentration is a strong prognostic factor for PTCL. For patients with elevated pre-treatment EBV-DNA, dynamic monitoring of EBV-DNA changes after initiation of therapy is useful for predicting treatment outcome and disease relapse.