Background: Serplulimab is a novel humanized monoclonal antibody against PD-1. At ASCO 2021, we have presented the results from the phase 2 serplulimab study (NCT03941574) in patients with unresectable/metastatic microsatellite instability-high or mismatch repair-deficient (MSI-H/dMMR) solid tumors who have progressed on or been intolerant to standard therapies with a median follow-up of 7.7 months. Here, we report the updated efficacy and safety results together with the results from sensitivity analysis after another 6-month follow-up. Methods: In this single-arm, open-label, multicenter, phase 2 study, patients aged 18–75 years with histologically or cytologically confirmed unresectable or metastatic MSI-H/dMMR solid tumors were enrolled to receive 3 mg/kg of intravenous serplulimab every two weeks for up to two years. The primary endpoint was objective response rate (ORR) assessed by an independent radiological review committee (IRRC) per RECIST v1.1. Secondary endpoints included ORR assessed by the investigators, duration of response (DoR), progression-free survival (PFS), overall survival (OS), and safety. Results: As of July 10, 2021, 108 patients had received at least one dose of study treatment and were included in the safety set (SS). Among them, 68 patients with confirmed MSI-H (by local sites or central lab) were included in the main efficacy analysis population (MEAP); 58 patients with confirmed MSI-H (by central lab) and had no major protocol deviations were included in the sensitivity analysis population (SAP). The median follow-up duration was 13.5 months in the MEAP and 14.0 months in the SAP. IRRC-assessed ORR per RECIST v1.1 was 39.7% (95% CI 28.0–52.3; 3 complete response [CR]) in the MEAP and 43.1% (95% CI 30.2–56.8; 2 CR) in the SAP. Investigator-assessed ORRs were 38.2% (95% CI 26.7–50.8; 1 CR) and 41.4% (95% CI 28.6–55.1; 1 CR) in the MEAP and the SAP, respectively. Median DoR, PFS, and OS were not reached; 12-month OS rate was 74.5% (95% CI 62.2–83.3) in the MEAP and 82.4% (95% CI 69.7–90.1) in the SAP. In the SS, 57 (52.8%) patients had grade ≥3 treatment-emergent adverse events, most commonly anemia (9.3%). Thirteen (12.0%) patients had grade ≥3 immune-related adverse events. Three (2.8%) deaths (2 progressive disease and 1 intestinal obstruction) that might be related to serplulimab were reported. Conclusions: The encouraging antitumor activity and the manageable safety profile sustained after a longer duration of follow-up, supporting the further development of serplulimab as a potential tissue-agnostic antitumor treatment. Clinical trial information: NCT03941574.