Iatrogenic diaphragmatic hernia is rare but potentially life-threatening and has rarely been reported after anterior thoracolumbar spinal surgery. We describe the case of an 80-year-old woman who developed an early postoperative left-sided diaphragmatic hernia following L1 vertebral body replacement (VBR) surgery, emphasizing the need for high vigilance and early diagnostic evaluation. The patient was initially diagnosed with an L1 osteoporotic vertebral fracture complicated by delayed union despite four months of conservative management, despite four months of conservative treatment, and she became unable to walk. Preoperative evaluation revealed a severely flattened L1 body, and computed tomography (CT) demonstrated posterior wall involvement. To address the persistent pain and mechanical instability, a two-stage procedure was performed: first, posterior fixation from T11 to L2, followed by L1 VBR through a left extrapleural approach with 10th-rib resection. A 5 cm pleural injury was identified and repaired intraoperatively. No obvious diaphragmatic injury was noted upon visual inspection of the surgical field. Chest radiography performed on postoperative day (POD) 1 was unremarkable. However, mild nausea appeared on POD2, which progressively worsened and led to a significantly reduced oral intake on POD6. Prompt CT evaluation demonstrated herniation of the stomach and spleen into the left thoracic cavity, confirming the diagnosis of an iatrogenic diaphragmatic hernia. On POD9, the patient underwent open abdominal repair by the surgical team. A 10-cm diaphragmatic defect was identified and closed with an expanded polytetrafluoroethylene (ePTFE; Gore-Tex) patch. At the 6-month follow-up, the diaphragmatic hernia had not recurred, and the patient's back pain had substantially improved, allowing her to walk independently with a cane. We speculate that an occult injury at the diaphragmatic attachment may have been masked by retraction or manipulation intraoperatively. The defect may then have enlarged due to postoperative increases in intra-abdominal pressure, presenting with gastrointestinal symptoms such as nausea. Iatrogenic diaphragmatic hernia must be included in the differential diagnosis when patients present with new or worsening respiratory or gastrointestinal symptoms after anterior thoracolumbar spinal surgery, and early CT is crucial for timely management of this condition.