Romidepsin is an important therapeutic option for patients with peripheral T-cell lymphoma (PTCL). However, the timing of romidepsin administration remains controversial. The objective of this study was to characterize the safety and efficacy of romidepsin as consolidation therapy after gemcitabine, dexamethasone, and cisplatin (GDP) therapy (GDPR). This study of patients treated between March 2019 and March 2021 was registered with the Japan Registry of Clinical Trials (registration number: jRCT0000000519). If complete response, partial response, or stable disease was confirmed after 2–4 GDP cycles, romidepsin was administered every 4 weeks for 1 year. Seven patients with relapsed/refractory (R/R) PTCL (T-follicular helper phenotype [n = 1] and angioimmunoblastic T-cell lymphoma [n = 6]) were included in this prospective study (PTCL-GDPR). After a median follow-up of 34 months of patients in PTCL-GDPR, the 2-year overall survival rate was 71%, and the overall response rate after treatment was 57%. Common adverse events in patients with PTCL-GDPR included hematological toxicities such as neutropenia, which improved with supportive treatment. There were no treatment-related mortalities. GDPR might be safe and effective in elderly transplant-ineligible patients with R/R PTCL; however, further investigation is required.