Background: Brachial plexus injury (BPI) is a peripheral nerve injury that results in severe functional impairment and disability. Even after prompt treatment, predicting the prognosis of BPI is not easy as it involves various factors. An objective and valid scoring system would aid clinicians in informing families and anticipating problems related to the recovery of BPI. Prognosis BPI (PRO-BPI) score, a new prognostic score to predict the outcome of traumatic BPI (TBPI), was developed in 2019 by Suroto and Rahman. This study aimed to evaluate its validity and reliability. Methods: Retrospective cohort analysis was conducted for 111 BPI patients. A serial assessment of Disabilities of the Arm, Shoulder and Hand (DASH) score and PRO-BPI score was done. Validity analysis was done by assessing Spearman correlations between PRO-BPI score and other scoring systems (DASH, Michigan hand outcomes, and 36-item short form survey score [SF-36]). Internal structure consistency using Cronbach’s alpha and test-retest reliability were measured for reliability analyses. A p-value was considered significant if < 0.05. Results: A total of 96 male and 15 female patients were included in our study with a mean age of 27.9 ± 10.6 years. Most of the patients (56.75%) had a poor prognosis based on the scoring system (average, 14.38 ± 3.98). Major contributors of this low score were the persistent pain (score 1 in 57.7% patients) and initial pain scale score (score 1 in 31.5% patients). Validity test showed that 6 parameters were all valid (p < 0.01). Reliability testing was done using Cronbach’s alpha and found acceptable internal consistency (α = 0.767). Test-retest reliability was high. Moderate correlations were observed between the measures. Conclusions PRO-BPI score is a valid and reliable scoring system in predicting the prognosis of TBPI.