Purpose This study aimed to assess sitting skills and trunk control in children with obstetric brachial plexus palsy (OBPP) and investigate the effect of these skills on upper extremity function. Methods A total of 106 children with OBPP aged 10-18 months were included in this study. Injury severity was determined with Narakas Classification. The Gross Motor Function Measurement Sitting Sub-scale was used to assess sitting skills, while the Sitting Assessment Scale was used to evaluate the trunk control. The upper extremity functions were assessed with using the Active Movement Scale and the Modified Mallet Score. Results As the severity of injury increased, sitting skills and trunk control values decreased (p= .0001). In addition, upper extremity function decreased with the decreasing sitting skills and trunk control (p= .0001). Conclusion Rehabilitation approaches should involve approaches that aim to increase trunk control in addition to programs targeting the extremity function in children with OBPP.