Background: Pucotenlimab is a humanized IgG4 anti-PD-1 monoclonal antibody with an engineered Fc domain. We aim to assess the efficacy and safety of pucotenlimab, and potential predictive biomarkers of response in patients with previously treated advanced microsatellite instability-high (MSI-H) or mismatch repair-deficient (dMMR) solid tumors. Methods: In this single-arm, multi-center, phase 2 study, eligible patients had locally advanced or metastatic, centrally confirmed MSI-H or dMMR solid tumors treated with at least one prior systemic therapy. Patients received pucotenlimab 200mg every 3 weeks until disease progression or unacceptable toxicity. The primary endpoint was overall response rate (ORR) assessed by the independent review committee (IRC) per Response Evaluation Criteria in Solid Tumors (RECIST) version 1.1. Besides, patients’ PD-L1 expression, blood neutrophil to lymphocyte ratio (NLR), germline HLA-I genotyping, loss of heterozygosity (LOH) of HLA-I genes, tumor mutational burden (TMB), and genetic alterations in the tumor revealed by next-generation sequencing were investigated to identify potential predictors of efficacy. Results: 100 patients with MSI-H or dMMR solid tumors were enrolled. Colorectal cancer (n=71) and gastric cancer (n=10) were the most common cancer types. The median follow-up duration was 22.5 months (range: 0.3-37.4) at data cutoff (December 4, 2021). The ORR was 49.0% (95% CI 38.86%-59.20%), and the 12-month progression-free survival (PFS) rate was 56.1% (95% CI 45.63%-65.24%) based on IRC’s assessment. Both the median PFS and median duration of response were not reached. Pucotenlimab was well tolerated. Grade ≥3 treatment-related adverse events (TRAEs) were reported in 18 patients, most commonly anemia, leukocytopenia, and hypertension. For the biomarker analysis, patients with NLR <4 at the second treatment cycle, TMB≥32.5 muts/Mb (the lower 35th TMB-percentile), or mutations in the KMT2D gene showed significantly higher ORR. Conclusions: Pucotenlimab demonstrated durable antitumor activity and manageable safety for previously treated patients with advanced MSI-H/dMMR solid tumors. KMT2D gene mutation, along with NLR and TMB, warrants further investigation as predictive biomarkers. Clinical trial information: NCT03704246.