Abstract Background Carpal tunnel syndrome (CTS) is a common cause of pain, weakness, sensory loss, and activity limitations. Currently, the most common initial treatment is use of a rigid splint immobilizing the wrist, usually during night-time, for several weeks. Evidence regarding the efficacy and effect durability of wrist splinting is weak. The treatment is associated with costs and may cause discomfort and limit daily and work activities. No placebo-controlled trials have been performed. Methods This is a randomized controlled trial designed to assess the efficacy of a rigid wrist splint compared with soft wrist bandage (placebo) in patients with primary idiopathic CTS. The trial will be conducted at an orthopedic department. Patients, 25 to 65 years old, who seek primary health-care with symptoms of CTS will be screened, and potentially eligible patients will be referred to the study center. Patients who fulfill the trial’s eligibility criteria will be invited to participate. A total of 112 patients who provide informed consent will be randomly assigned to treatment with either a rigid wrist splint or a soft bandage to be used initially for 6 weeks at night and, if possible, during the day. The splints and bandages will be fitted with a temperature-monitoring device to measure the total time during which they have actually been worn. The trial participants will complete a questionnaire that includes the 6-item CTS symptoms scale (CTS-6); the 11-item disabilities of the arm, shoulder, and hand (QuickDASH) scale; and the EuroQol 5-dimension (EQ-5D) health status and quality-of-life measure at baseline and at 6, 12, 24, and 52 weeks after treatment start. The participants will undergo physical examination and nerve conduction testing at baseline and at 52 weeks. The trial’s primary outcomes are the change in the CTS-6 score from baseline to 12 weeks and the rate of carpal tunnel release surgery at 52 weeks. Discussion This is the first placebo-controlled randomized trial with electronic monitoring of actual splint use and will provide evidence regarding the efficacy of wrist splinting in patients with CTS. Trial registration ISRCTN Registry, ISRCTN81836603. Registered on May 5, 2018.