Hepatocellular carcinoma (HCC), particularly when accompanied by microvascular invasion (MVI), has a markedly high risk of recurrence after liver resection. Adjuvant immunotherapy is considered a promising avenue. This multicenter, open-label, randomized, controlled, phase 2 trial was conducted at six hospitals in China to assess the efficacy and safety of adjuvant sintilimab, a programmed cell death protein 1 inhibitor, in these patients. Eligible patients with HCC with MVI were randomized (1:1) into the sintilimab or active surveillance group. The sintilimab group received intravenous injections every 3 weeks for a total of eight cycles. The primary endpoint was recurrence-free survival (RFS) in the intention-to-treat population. Key secondary endpoints included overall survival (OS) and safety. From September 1, 2020, to April 23, 2022, a total of 198 eligible patients were randomly allocated to receive adjuvant sintilimab (n = 99) or undergo active surveillance (n = 99). After a median follow-up of 23.3 months, the trial met the prespecified endpoints. Sintilimab significantly prolonged RFS compared to active surveillance (median RFS, 27.7 versus 15.5 months; hazard ratio 0.534, 95% confidence interval 0.360-0.792; P = 0.002). Further follow-up is needed to confirm the difference in OS. In the sintilimab group, 12.4% of patients experienced grade 3 or 4 treatment-related adverse events, the most common of which were elevated alanine aminotransferase levels (5.2%) and anemia (4.1%). These findings support the potential of immune checkpoint inhibitors as effective adjuvant therapy for these high-risk patients. Chinese Clinical Trial Registry identifier: ChiCTR2000037655. Results from a multicenter, randomized phase 2 trial in China show that adjuvant anti-PD-1 therapy in patients with resected hepatocellular carcinoma with microvascular invasion leads to prolonged recurrence-free survival compared to active surveillance.