Objectives: To examine the effect of an early postsurgical intervention consisting of graded activity and pain education (GAPE) in patients with chronic low back pain (CLBP) undergoing lumbar spinal fusion (LSF) on sedentary behavior, disability, pain, fear of movement, self-efficacy for exercise and health-related quality of life (HRQoL) at 3-, 6-, and 12 months follow-up. Design: A parallel-group, observer-blinded randomized controlled trial. Setting: Department of Occupational- and Physiotherapy and the Centre for Rheumatology and Spine Diseases, Rigshospitalet, Denmark. Participants: In total, 144 participants undergoing an LSF for CLBP were randomly assigned to an intervention or a control group. Interventions: The intervention group received 9 sessions of GAPE, based on principles of operant conditioning. Main Outcome Measures: The primary outcome was reduction in time spent in sedentary behavior, measured by an accelerometer at 3 months. The secondary outcomes were reduction in time spent in sedentary behavior at 12 months and changes from baseline to 3-, 6-, and 12 months on disability, pain, fear of movement, self-efficacy for exercise, and HRQoL. Results: No difference in changes in sedentary behavior between groups was found 3 months after surgery. At 12 months after surgery, there was a significant difference between groups (mean difference: −25.4 min/d (95% confidence interval −49.1 to −1.7)) in favor of the intervention group. Conclusions: Compared with usual care, GAPE had no effect on short-term changes in sedentary behavior but GAPE had a statistical, but possibly not clinical significant effect on sedentary behavior 12 months after LSF. Further, the behavioral intervention was safe to perform.