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Gastric adenocarcinoma characterized by deficient mismatch repair (dMMR)/high microsatellite instability (MSI-H) represents a distinct subtype with high tumor immunogenicity and inherent sensitivity to immune checkpoint inhibitors. While neoadjuvant dual immunotherapy or chemoimmunotherapy has demonstrated high pathological complete response (pCR) rates in locally advanced dMMR/MSI-H gastric cancer, evidence supporting neoadjuvant monotherapy with Cadonilimab remains scarce. Cadonilimab is a PD-1/CTLA-4 bispecific antibody. We herein present the case of a 72-year-old man with locally advanced gastric antral adenocarcinoma (cT3N0M0, dMMR/MSI-H, PD-L1 CPS = 30) who received three cycles of neoadjuvant Cadonilimab (625 mg, Q3W) followed by radical surgery. Postoperative pathological assessment confirmed a pCR (TRG0), with no residual tumor identified in the gastric specimen or any of the 32 dissected lymph nodes. This case offers compelling evidence that Cadonilimab monotherapy can induce a profound pathological response with a favorable safety profile, supporting its further investigation as a personalized neoadjuvant strategy for this disease subset.