Objective: The purpose of this study was to investigate the effects of a short thumb opponens splint on the upper extremity and hand functions in a child with an obstetric brachial plexus injury. Methods: This single-subject experimental design study of an 11 year-old male patient diagnosed with an obstetric brachial plexus injury was conducted using the ABA’ design. A total of 20 sessions were held, once a week for 20 weeks. During the baseline period, five sessions were assessed without intervention. During the intervention period, ten sessions were assessed after intervention with a 6 h short thumb opponens splint. In the follow-up period, five sessions were assessed without intervention. To measure the upper extremity and hand functions, a Box and Block Test(BBT), nine-hole peg test, mallet scale, and an active movement scale were assessed before, during, and after intervention. Results: After intervention with a short thumb opponens splint, the participant showed significant improvement in hand(BBT, nine-hole peg test) and upper extremity(mallet scale, active movement scale) functions and demonstrated improvement. Conclusion: The results of this study suggest that use of a short thumb opponens splint has a positive effect on the upper extremity and hand functions. These results can be used to form basis of a clinical intervention in patients with an obstetric brachial plexus injury.