This study aims to verify the feasibility and efficacy of laparoscopic lower mediastinal lymphadenectomy for Siewert type II/III adenocarcinoma of esophagogastric junction (AEG). An exploratory, observational, prospective, cohort study will be carried out under the Idea, Development, Exploration, Assessment and Long-term Follow-up (IDEAL) framework (stage 2b). The study will recruit 1,036 patients with cases of locally advanced AEG (Siewert type II/III, clinical stage cT2-4aN0-3M0), and 518 will be assigned to either the laparoscopy group or the open group. Patients will receive lower mediastinal lymphadenectomy along with either total or proximal gastrectomy. The primary endpoint is the number of lower mediastinal lymph nodes retrieved, and the secondary endpoints are the surgical safety and prognosis, including intraoperative and postoperative lower-mediastinal-lymphadenectomy-related morbidity and mortality, rate of rehospitalization, R0 resection rate, 3-year local recurrence rate, and 3-year overall survival. The study will provide data for the guidance and development of surgical treatment strategies for AEG. The study has been registered in ClinicalTrials.gov (No. NCT04443478).