Objective: To investigate the effects that wearing unstable shoes has on disability, trunk muscle activity, and lumbar spine range of motion (ROM) in patients with chronic lower back pain (CLBP). Design: Randomized controlled trial. Setting: Orthopedic Surgery Service. Participants: We randomized 40 adults with nonspecific CLBP either to an unstable shoes group (n = 20) or to the control group (n = 20). Intervention: The participants in the unstable shoes group were advised to wear these shoes for a minimum of six hours a day for four weeks. Control group participants were asked to continue wearing their regular shoes. Outcome measures: Our primary outcome was measurement of back-related dysfunction, assessed using the Roland-Morris Disability Questionnaire. Secondary outcomes included changes in electromyographic (EMG) activity of erector spinae (ES), rectus abdominis (RA), internus obliquus (IO), and externus obliquus (EO) muscles, and changes in lumbar spine ROM. Results: Between-group analysis highlighted a significant decrease in disability in the unstable shoes group compared to the control (−5, 95% confidence interval (CI) = −8.4 to −1.6). Our results revealed a significant increase in the percentage of RA, ES, IO, and EO EMG activity and in lumbar spine ROM in the unstable shoes group compared to the control group. Moreover, our results showed a significant negative correlation between disability and the percentage of ES, RA, and IO muscle activity at the end of the intervention.