Abstract Background Patients with chronic low back pain undergoing lumbar spinal fusion (LSF) are physically inactive and thereby at risk of poor health. Barriers to being physically active need to be acknowledged in post-surgical rehabilitation. The primary objective of this randomized controlled trial (RCT) is to examine the effect of an early active intervention consisting of graded activity and pain education (GAPE) on sedentary behaviour in a population of patients undergoing LSF. The secondary objective is to examine the effect of GAPE on disability, pain, fear of movement, self-efficacy for exercise, and health-related quality of life. Methods The study is an RCT planned to include 144 patients undergoing LSF at 1–2 levels for low back pain caused by degeneration of the lumbar spine. The patients will be randomly assigned to receive either usual care or usual care plus GAPE. GAPE consists of nine individual physiotherapist-guided sessions over a 10-week period. The overall purpose is to reduce sedentary behaviour, by educating the patient about pain and, based on a cognitive behavioural perspective, gradually strengthen the patient’s self-efficacy to be physically active and reduce fear of movement. The physiotherapist will plan the intervention in collaboration with the patient. Based on a semi-structured interview and observations of the patient in their home, they will set individually functional goals. The primary outcome will be a reduction in sedentary behaviour, measured by an accelerometer at baseline (pre-surgery) and at 3 and 12 months post-surgery. Secondary outcomes will include disability, pain, fear of movement, self-efficacy for exercise, and quality of life. Secondary outcome data will be collected at baseline (pre-surgery) and at 3, 6 and 12 months post-surgery. Discussion We hypothesize that, compared with the “usual care group”, GAPE will primarily lead to a significant reduction in sedentary behaviour, and secondarily a reduction in disability, pain intensity, and fear of movement; further, it will increase the patient’s self-efficacy for exercise and quality of life. Trial registration www.clinicaltrials.gov NCT04103970, Registered on 24 September 2019