Though several anti-PD-1/PD-L1 antibodies approved for monotherapy in microsatellite instability-high or mismatch repair-deficient unresectable/metastatic solid tumors, novel immunotherapy with better anti-tumor activity is needed in clinic. In this single-arm, multicenter, pivotal, phase II study, patients received iparomlimab (a novel humanized anti-PD-1 mAb, 200 mg or 3 mg/kg for patients with body weight < 40 kg, IV, Q3W) until disease progression, intolerable toxicities, withdrawal of consent, death, or up to 2 years. The primary endpoint was objective response rate (ORR) assessed by independent radiological review committee (IRRC). Totally, 120 patients were enrolled, of whom 60 patients failed from prior standard therapy, were enrolled in the full analysis set (FAS). As of Jan 20, 2024, the confirmed ORR per IRRC in FAS were 50.0% (30/60; 95% CI 36.8-63.2%) patients, including 4 (6.7%) complete response (CR) and 26 (43.3%) partial response (PR). In colorectal cancer (CRC) patients in FAS, the ORR reached 57.9% (22/38; 95% CI 40.8-73.7%) per IRRC, with 3 (7.9%) CR and 19 (50.0%) PR. Furtherly, the ORRs in liver metastatic or non-liver metastatic CRC patients were 52.9% (9/17, 95% CI 27.8-77.0%) vs 61.9% (13/21, 95% CI 38.4%-81.9%). The incidence of TRAE was 90.8% (any grade) and 20.8% (grade >= 3). Immune-related adverse events occurred in 33.3% (any grade) and 5.0% (grade >= 3) of patients. No iparomlimab-related death occurred. Iparomlimab presented encouraging antitumor activity with durable response and tolerable safety profile. Trial registration ClinicalTrials.gov Identifier: NCT04326829.