Background <p>The incidence of incidental gallbladder cancer (IGBC) has been reported to be approximately 1% in patients undergoing cholecystectomy.^{<CitationRef CitationID="CR1">1</CitationRef>} The guidelines recommend additional lymph node dissection for IGBC invading the subserosal layer or deeper;^{<CitationRef CitationID="CR2">2</CitationRef>} however, two-stage surgery is expected to involve abdominal adhesions, and lymph node dissection of the pancreatic head dorsal area is technically complicated because of poor views and the necessity of colon mobilization in cases using the conventional intra-abdominal approach. The retroperitoneal-first laparoscopic approach (Retlap) was reported by Kiguchi et al. as a minimally invasive surgical procedure for advanced pancreatic body cancer, which is not affected by the intraperitoneal situation.^{<CitationRef CitationID="CR3">3</CitationRef>,<CitationRef CitationID="CR4">4</CitationRef>} Applying this technique, we performed Retlap-assisted additional lymph node dissection for IGBC.</p> Methods <p>A 59-year-old woman underwent laparoscopic cholecystectomy for symptomatic cholelithiasis, and IGBC was diagnosed based on a pathological examination. The depth of invasion was T3a, and additional lymph node dissection was performed using Retlap.</p> Results <p>The operation time was 326&#xa0;min, the retention time was 153&#xa0;min, and the estimated blood loss was 30&#xa0;mL. The patient was discharged on the sixth postoperative day without any complications. Pathologically, 13 lymph nodes were dissected but there were no malignant findings.</p> Conclusion <p>Retlap-assisted additional lymph node dissection for IGBC is not affected by intra-abdominal adhesions, provides a good visual field, does not require mobilization of the colon, and allows minimally invasive surgery. Since the robotic approach for IGBC has already been safely implemented,^{<CitationRef CitationID="CR5">5</CitationRef>} we aim to apply the robotic approach when using Retlap in the future.</p>