Abstract Tisagenlecleucel demonstrated high response rates and a manageable safety profile in adults with relapsed/refractory diffuse large B-cell lymphoma (r/r DLBCL) in the JULIET trial. However, lack of response and chimeric antigen receptor (CAR) T-cell exhaustion were observed in patients with programmed cell death protein 1 (PD-1) overexpression. Hence, pembrolizumab, a PD-1 inhibitor, was hypothesized to improve efficacy and cellular expansion of CAR T-cells in vivo. Here, we report the final analysis of the PORTIA trial in adult patients with r/r DLBCL who had ≥2 prior lines of therapy and had an Eastern Cooperative Oncology Group performance status of ≤1. Patients received 1 tisagenlecleucel infusion on day 1. Pembrolizumab (200 mg) was given every 21 days, for up to 6 doses. Three cohorts initiated pembrolizumab on days 15 (n = 4), 8 (n = 4), or –1 (n = 4). Safety, efficacy, cellular kinetics, and biomarker analyses were included. Tisagenlecleucel plus pembrolizumab was feasible and showed a manageable safety profile, without dose-limiting toxicities. Emerging efficacy with tisagenlecleucel was observed when pembrolizumab was given the day before tisagenlecleucel; however, the limited patient sample and short follow-up do not allow for definitive conclusions. Adding pembrolizumab to tisagenlecleucel did not augment the cellular expansion of tisagenlecleucel but delayed peak expansion if given the day before tisagenlecleucel (NCT03630159).