This is a prospective, multicenter, randomized, phase II clinical trial intended to enroll patients with HER2-moderate/high-expressing, pathologically staged stage III gastric cancer who have undergone D2 or more extensive surgery. The study aims to evaluate the preliminary efficacy and safety of disitamab vedotin combined with the SOX regimen versus SOX alone as post-operative adjuvant therapy.

After providing informed consent and meeting all eligibility criteria, participants will begin adjuvant therapy with disitamab vedotin plus chemotherapy (SOX) approximately four weeks after surgery, continuing for 6-8 cycles. Post-operative imaging assessments will be performed every three months until disease recurrence. Following recurrence, survival follow-up will be conducted every three months. Safety visits will continue from first drug administration until 60 days after the last dose or until initiation of new anti-tumor therapy.

Inclusion Criteria:

• Aged between 18 and 75 years.
• Have undergone radical resection with D2 or wider lymph node dissection and achieved R0 resection status.
• No prior systemic anti-tumor treatment (i.e., neoadjuvant therapy) before surgery.
• Histopathologically confirmed gastric adenocarcinoma.
• Pathological stage III gastric and gastroesophageal junction adenocarcinoma patients (according to the 8th edition of the American Joint Committee on Cancer [AJCC] staging system).
• HER2 moderate-to-high expression (IHC 3+ or 2+).
• Eastern Cooperative Oncology Group Performance Status (ECOG PS) of 0 or 1.
• Expected survival time ≥ 3 months.
• Able to understand the study protocol, voluntarily participate in the study, and provide written informed consent;
• Good compliance and able to cooperate with the treatment regimen specified in this study;
• Possesses complete imaging and pathological clinical data;
• Adequate organ and bone-marrow function.

Exclusion Criteria:

• Primary stage III gastric or gastroesophageal-junction adenocarcinoma not histologically or cytologically confirmed;
• Unable to receive disitamab vedotin or SOX chemotherapy;
• Unable to comply with the required follow-up schedule;
• Unable to accept the treatment regimen specified in this protocol;
• Unable or unwilling to undergo mandated response assessments (e.g., CT imaging);
• Active autoimmune disease;
• History of psychoactive-substance abuse that cannot be discontinued, or any severe/uncontrolled psychiatric disorder, or any severe/uncontrolled systemic disease;
• Any concomitant condition that, in the investigator's opinion, poses significant risk to the subject or could compromise study completion;
• Other malignancies within 5 years before screening, except adequately treated cancers considered cured (e.g., thyroid cancer, cervical carcinoma in situ, basal/squamous-cell skin cancer, or ductal carcinoma in situ of the breast treated with curative surgery);
• Lactating women;
• Prior neoadjuvant therapy or intra-operative intraperitoneal chemotherapy.