Photobiomodulation therapy (PBMT) has been used in several musculoskeletal disorders to reduce pain, inflammation and promoting tissue regeneration. The current evidence about the effects of PBMT on low back pain are still conflicting. We aimed to evaluate the effects of photobiomodulation therapy against placebo on pain intensity and disability in patients with chronic non-specific low back pain. This was a prospectively registered, randomised placebo-controlled trial, with blinded patients, therapists and assessors. The study was conducted on an outpatient physical therapy clinic in Brazil, between April 2017 and May 2019. A total of 148 patients with chronic non-specific low back pain were randomised to either active photobiomodulation therapy (n=74) or placebo (n=74). Patients from both groups received 12 treatment sessions, 3 times a week, for 4 weeks. Patients from both groups also received an educational booklet based on 'The Back Book'. Clinical outcomes were measured at baseline and at follow-up appointments at 4 weeks, 3, 6 and 12 months after randomization. The primary outcomes were pain intensity and disability measured at 4 weeks. We estimated the treatment effects using linear mixed models following the principles of intention to treat. There was no clinical important between-group differences in terms of pain intensity (Mean Difference=0.01 point; 95% CI=-0.94 to 0.96) and disability (Mean Difference=-0.63 points; 95% CI=-2.23 to 0.97) at 4 weeks. Patients did not report any adverse events. Photobiomodulation therapy was not better than placebo to reduce pain and disability in patients with chronic non-specific low back pain.