The study will evaluate the progression-free survival benefit of duvelisib monotherapy as compared to investigator's choice of gemcitabine or bendamustine in participants with relapsed/refractory nodal T cell lymphoma with TFH phenotype.

Key Inclusion Criteria:

• Pathologically confirmed nodal T cell lymphoma with TFH phenotype according to the criteria of the World Health Organization classification (Swerdlow 2017, Alaggio 2022) including any one of Angioimmunoblastic T cell lymphoma (AITL), follicular T cell lymphoma, and other nodal peripheral T cell lymphoma (PTCL) with a TFH phenotype.
• Relapsed or refractory to at least 1 prior systemic, cytotoxic therapy for T cell lymphoma.
• Measurable disease as defined by Lugano 2014 criteria (Cheson 2014) for T cell lymphoma.

Key Exclusion Criteria:

• Cutaneous-only disease.
• Received prior allogeneic transplant any time in the past or received autologous transplant within 60 days prior to the first dose of study drug.
• Received prior treatment with a phosphoinositide-3-kinase (PI3K) inhibitor.
• Prior exposure to planned study treatment investigator's choice therapy (gemcitabine or bendamustine) within 60 days prior to the first dose of study drug.

Other protocol-defined criteria apply.