Chronic low back pain (CLBP) is the most expensive cause of workrelated disability: it causes the highest number of years lived with disability. The most severe and debilitated CLBP patients often have comorbidities such as overweight and obesity. Despite the growing body of scientific literature pointing towards the close interaction between overweight/obesity and CLBP, few treatment programs for people with CLBP nowadays take overweight into account. Therefore this study will examine the added value of a behavioral weight reduction program (changes in diet, behavior and physical exercise) to current best evidence rehabilitation (pain neuroscience education plus cognition-targeted exercise therapy) for overweight or obese people with CLBP. An international, multicenter randomized controlled trial comparing a behavioral weight reduction program combined with pain neuroscience education and cognition-targeted exercise therapy versus pain neuroscience education and cognition-targeted exercise therapy alone, will be conducted. The primary outcome is pain and the primary endpoint was chosen at 12 months follow-up; secondary outcomes include health care use and daily functioning (see detailed description of outcomes for an overview of all secondary outcomes). If the promising results of the proof of concept study are corroborated, the new intervention will have a high socio-economic impact, including an annual health care cost reduction of €66 million in Switzerland, and €60 million in Flanders, and is expected to increase life expectancy in the long term.