Objective: We aimed to explore the feasibility of a single-port thoracoscopy- assisted five-step laparoscopic procedure via transabdominal diaphragmatic(TD) approach(abbreviated as five-step maneuver) for No.111 lymphadenectomy in patients with Siewert type II esophageal gastric junction adenocarcinoma (AEG). Methods: This was a descriptive case series study. The inclusion criteria were as follows: (1) age 18-80 years; (2) diagnosis of Siewert type II AEG; (3) clinical tumor stage cT2-4aNanyM0; (4) meeting indications of the transthoracic single-port assisted laparoscopic five-step procedure incorporating lower mediastinal lymph node dissection via a TD approach; (5) Eastern Cooperative Oncology Group performance status (ECOG PS) 0-1; and (6) American Society of Anesthesiologists classification I, II, or III. The exclusion criteria included previous esophageal or gastric surgery, other cancers within the previous 5 years, pregnancy or lactation, and serious medical conditions. We retrospectively collected and analyzed the clinical data of 17 patients (age [mean ± SD], [63.6±11.9] years; and 12 men) who met the inclusion criteria in the Guangdong Provincial Hospital of Chinese Medicine from January 2022 to September 2022. No.111 lymphadenectomy was performed using five-step maneuver as follows: superior to the diaphragm, starting caudad to the pericardium, along the direction of the cardio-phrenic angle and ending at the upper part of the cardio-phrenic angle, right to the right pleura and left to the fibrous pericardium , completely exposing the cardio-phrenic angle. The primary outcome includes the numbers of harvested and of positive No.111 lymph nodes. Results: Seventeen patients (3 proximal gastrectomy and 14 total gastrectomy) had undergone the five-step maneuver including lower mediastinal lymphadenectomy without conversion to laparotomy or thoracotomy and all had achieved R0 resection with no perioperative deaths. The total operative time was (268.2±32.9) minutes, and the lower mediastinal lymph node dissection time was (34.0±6.0) minutes. The median estimated blood loss was 50 (20-350) ml. A median of 7 (2-17) mediastinal lymph nodes and 2(0-6) No. 111 lymph nodes were harvested. No. 111 lymph node metastasis was identified in 1 patient. The time to first flatus occurred 3 (2-4) days postoperatively and thoracic drainage was used for 7 (4-15) days. The median postoperative hospital stay was 9 (6-16) days. One patient had a chylous fistula that resolved with conservative treatment. No serious complications occurred in any patient. Conclusion: The single-port thoracoscopy-assisted five-step laparoscopic procedure via a TD approach can facilitate No. 111 lymphadenectomy with few complications.