Background For patients with synchronous liver and lung metastases from colorectal cancer, the invasiveness of adding thoracic to abdominal surgery is an obstacle to concurrent liver and lung metastasectomy. We developed a simple technique to resect lung lesions via a transdiaphragmatic approach without thoracic incision in patients undergoing liver metastasectomy. Methods Sixteen patients with synchronous liver and unilateral lung metastases underwent transdiaphragmatic wedge resection of lung lesions simultaneous with liver metastasectomy. Short-term operative outcomes were compared with those in 102 patients treated with conventional unilateral wedge resection for colorectal lung metastases. Results Twenty peripheral (<3 cm from the pleura) lung lesions from various locations in the lung were resected via transdiaphragmatic approach. No conversions to conventional approach were required. The median tumor number and size were 1 (range, 1–3) and 8 mm (range, 3–30 mm), respectively. Transdiaphragmatic resection reduced median operative blood loss compared with conventional resection (0 mL vs 50 mL [P < .001]) and reduced median duration of hospital stay compared with staged liver and lung resection (6 days vs 11 days [P < .001]). Operative duration and rates of lung-related morbidity and positive surgical margin were similar between the transdiaphragmatic and conventional groups (104 minutes vs 105 minutes [P = .61], 13% vs 4% [P = .15], and 6% vs 5% [P = .73], respectively). Conclusion Simultaneous transdiaphragmatic resection of peripheral lung lesions is safe in patients undergoing liver resection. The low-invasive transdiaphragmatic approach facilitates aggressive operative treatment for synchronous liver and lung metastases.