Functional limitation of the upper extremity (UE) in obstetrical brachial plexus paralysis (OBPP) restricts a child's participation in daily living and social activities. In treatment, the participation of UE in rehabilitation is important. Constraint-induced movement therapy (CIMT) is a promising rehabilitation approach that is used to improve the UE functions of patients with neurological dysfunctions. This single-blinded randomized controlled clinical trial includes 30 pediatric patients diagnosed with chronic OBPP aged between 2 and 12 years. The patients were divided into two groups as a modified CIMT group and a control group. Patients in both groups underwent classical rehabilitation treatment 4 times a week for 8 weeks. Range of motion (ROM), stretching, strengthening, and proprioceptive exercises were given to both control and CIMT group. The patients in the CIMT group had to wear constraining arm slings 2 h per day and 4 days a week for 8 weeks. The patients were evaluated both before and after treatment using the Mallet classification system and the Melbourne unilateral upper limb assessment-2 (The MA2) scale. In both groups, the Mallet and MA2 scores significantly increased after the treatment process. However, the percentage of improvement was higher for the CIMT group. Modified CIMT improves the joint ROM and the functional use of the extremity among OBPP-diagnosed children. This improvement is greater in the CIMT group compared to the improvement in the control group. Implementation of CIMT in a routine rehabilitation process may be helpful.