Abstract Background There is no consensus regarding optimal treatment for peripheral T-cell lymphomas (PTCL), especially in relapsed or refractory cases, which have very poor prognosis and a dismal outcome, with 5-year overall survival of 30 %. Methods A multicenter prospective phase II trial was conducted to investigate the role of the combination of gemcitabine plus romidepsin (GEMRO regimen) in relapsed/refractory PTCL, looking for a potential synergistic effect of the two drugs. GEMRO regimen contemplates an induction with romidepsin plus gemcitabine for six 28-day cycles followed by maintenance with romidepsin for patients in at least partial remission. The primary endpoint was the overall response rate (ORR); secondary endpoints were survival, duration of response, and safety of the regimen. Results The ORR was 30 % (6/20) with 15 % (3) complete response (CR) rate. Two-year overall survival was 50 % and progression-free survival 11.2 %. Grade ≥3 adverse events were represented by thrombocytopenia (60 %), neutropenia (50 %), and anemia (20 %). Two patients are still in CR with median response duration of 18 months. The majority of non-hematological toxicities were mild and transient. No treatment-related death occurred and no toxicity led to treatment interruption. Conclusions GEMRO combination regimen shows efficacy data similar to those of single-agent romidepsin with additional hematologic toxicities. Synergy observed in preclinical phase did not turn into ability to improve clinical outcomes. Trial registration The trial was registered under EudraCT 2012-001404-38; ClinicalTrials.gov number, NCT01822886 .