• Inclusion Criteria
• 1. Patients aged ≥ 18 years at time of screening
• 2. Written informed consent by the patient or the patients' legally authorized representative prior to screening
• 3. Patients with histologically or cytologically confirmed diagnosis of one of the following subtypes of T-cell lymphoma:
• 1. Peripheral T-cell lymphoma (PTCL): Angioimmunoblastic T-cell lymphoma (AITL), PTCL, not otherwise specified (PTCL-NOS), nodal PTCL with T-follicular helper (TFH) and follicular T-cell lymphoma (FTCL)
• 2. Cutaneous T-cell lymphoma (CTCL) (stages II-B, III, and IV): Mycosis fungoides (MF) and Sezary syndrome (SS)
• 4. Patients must have received at least 2 prior systemic therapies
• 5. Patients with PTCL must have at least 1 measurable lesion (Cheson BD, 2014)
• 6. Patients with CTCL must have assessable disease by response criteria for CTCL (Olsen EA, 2011)
• 7. Eastern Cooperative Oncology Group Performance Status (ECOG PS) = 0-2
• 8. Life expectancy of at least 3 months
• 9. Adequate bone marrow, renal and hepatic functions

Exclusion Criteria:

• 1. Patients with central nervous system (CNS) involvement
• 2. Patients with Adult T-cell leukemia/lymphoma (ATLL)
• 3. Prior allogeneic stem cell transplant
• 4. Prior treatment with ONO-4685, anti-PD-1, anti-PD-L1, anticytotoxic T lymphocyte associated protein 4 (CTLA-4) antibody, or any other antibody or drug specifically targeting T-cell co-stimulation or checkpoint pathways
• 5. Prior allogeneic and autologous chimeric antigen receptor (CAR) T-cell therapy
• 6. Patients with malignancies (other than T-cell lymphoma) except for completely resected basal cell carcinoma, stage I squamous cell carcinoma, carcinoma in situ, or any other malignancies that has not relapsed for at least 2 years
• 7. History of severe allergy or hypersensitivity to any monoclonal antibodies, other therapeutic proteins or corticosteroid (e.g., dexamethasone)
• 8. History of infection with Mycobacterium tuberculosis within 2 years prior to the first dose of study treatment
• 9. Patients with systemic and active infection including human immunodeficiency virus (HIV), hepatitis B or C virus infection
• 10. Patients not recovered to Grade 1 or stabilized from the adverse effects (excluding alopecia) of any prior therapy for their malignancies
• 11. Women who are pregnant or lactating