Background: Pembrolizumab monotherapy showed durable antitumor activity and a manageable safety profile in patients with sorafenib-treated (cohort 1) and treatment-naive (cohort 2) aHCC in the open-label, phase 2 KEYNOTE-224 (NCT02702414) study. Longer term data from KEYNOTE-224 after ̃3 years of follow-up for patients with treatment-naive aHCC are reported. Methods: Eligible patients in cohort 2 had histologically, cytologically, or radiologically confirmed aHCC, Barcelona Clinic Liver Cancer stage C or B not amenable or refractory to locoregional therapy and not amenable to curative treatment, Child-Pugh A liver function, measurable disease per RECIST v1.1 by blinded independent central review (BICR), and ECOG PS 0 or 1. Patients received pembrolizumab 200 mg intravenously every 3 weeks for ≤35 cycles (̃2 years). Primary end point was ORR assessed per RECIST v1.1 by BICR. Secondary end points included DOR, DCR, TTP, and PFS, all assessed per RECIST v1.1 by BICR, OS, and safety/tolerability. Results: All 51 patients enrolled in cohort 2 received ≥1 dose of pembrolizumab. Median follow-up, defined as the time from first dose to the data cutoff (October 1, 2021), was 35 months (range, 31-37). ORR was 16% (95% CI, 7-29). Median DOR was not reached (NR; range, 3 to 24+ months); 58% of responders were estimated to have a response duration ≥18 months. Best overall response was 8 (16%) PRs, 21 (41%) SDs, and 17 (33%) PDs; no CRs were observed and response was not evaluable for 2 patients (4%) and not assessed for 3 patients (6%). DCR was 57% (95% CI, 42-71). ORR was generally consistent among patients with a viral and nonviral etiology for HCC, although sample sizes were small. The median TTP was 4 months (95% CI, 3-9). Median PFS was 4 months (95% CI, 2-8). Estimated PFS rate at 24 months was 15%. Median OS was 17 months (95% CI, 8-23). Estimated OS rate at 24 months was 34%. No new or unexpected adverse events (AEs) occurred. Treatment-related AEs were reported in 28 patients (55%; grade 3-5, 8 [16%]). Conclusions: Updated results from cohort 2 of the KEYNOTE-224 study continued to demonstrate durable antitumor activity, promising OS, and manageable safety for pembrolizumab monotherapy in patients with aHCC and no prior systemic therapy. These data, together with recent positive results from KEYNOTE-394, underscore the broad applicability of pembrolizumab in patients with aHCC both as monotherapy and in combination with other therapies. Clinical trial information: NCT02702414.