Adult T-cell leukemia/lymphoma (ATL) is a peripheral T-cell malignancy with a poor prognosis. We conducted a retrospective study across six institutions in Miyazaki Prefecture, Japan, to assess the efficacy of tucidinostat in patients with relapsed/refractory ATL who had not undergone transplantation. Between October 2021 and July 2023, 24 patients aged 41 to 88 years (median, 73.4 years) who had undergone prior therapies, including intensive chemotherapy (79.2%) and mogamulizumab immunotherapy (79.2%), received tucidinostat. Objective response rate (ORR), disease control rate (DCR), progression-free survival (PFS), and overall survival (OS) were evaluated as key outcomes. ORR and DCR reached 54.2% and 91.7%, respectively. The median PFS was 3.95 months, and OS was 8.04 months, which were not inferior to the results of a phase IIb study. The influential factors for PFS were age ≥ 75 years and high soluble IL-2 receptor (sIL-2R) levels above 5000 U/mL at the start of treatment. Favorable patients without these factors achieved a PFS of 11.4 months. Treatment-related adverse events were mainly hematologic but were managed over the course of treatment. Our findings indicate that tucidinostat provides survival benefits in patients with relapsed/refractory ATL in clinical practice and highlight key clinical factors for better outcomes.