BACKGROUND: After lumbar spine surgery, a Core Outcome Set (COS) for acute pain is essential to ensure that the most meaningful outcomes are monitored consistently in the perioperative period. The aim of the present study was to consent on a COS for assessing the efficacy of acute pain management for patients undergoing lumbar spinal surgery. METHOD: A modified Delphi procedure was conducted among a national (Dutch) expert panel. External endorsement of the final COS was conducted among an international panel of anaesthesiologists and the Dutch chronic pain patient association. RESULTS: A panel of 35 experts representing 10 stakeholder groups, including orthopaedic surgeons, anaesthesiologists, patient representatives, physician assistants, researchers, a neurosurgeon, nurses, and a psychologist, took part in the Delphi procedure. Five outcome domains reached consensus for inclusion in this COS. This COS contains the following domains: pain intensity, analgesic use, early mobilisation, length of stay, and adverse events. Of an international panel of 27 key opinion leaders, 77% agreed on the final COS. The patient association also consented to the final COS. CONCLUSIONS: A COS to evaluate acute pain treatment after lumbar surgery is proposed after national Delphi consensus rounds and (international) external endorsement. Future research should focus on determining suitable measurement instruments, assessing feasibility, validation, and implementation of the COS in daily clinical practice and research. SIGNIFICANCE: This research proposes a clinically relevant spine-specific core outcome set (COS) of domains focusing on the acute postoperative phase (until 30 days). This is the first COS for evaluation of acute pain after lumbar spine surgery.