Background Traumatic brachial plexus injuries affect 1% of patients involved in major trauma. MRI is the best test for traumatic brachial plexus injuries, although its ability to differentiate root avulsions (which require urgent reconstructive surgery) from other types of nerve injury remains unknown. Purpose To evaluate the accuracy of MRI for diagnosing root avulsions in adults with traumatic brachial plexus injuries. Materials and Methods For this systematic review, MEDLINE and Embase were searched from inception to August 20, 2018. Studies of adults with traumatic nonpenetrating unilateral brachial plexus injuries were included. The target condition was root avulsion. The index test was preoperative MRI, and the reference standard was surgical exploration. A bivariate meta-analysis was used to estimate summary sensitivities and specificities of MRI for avulsion. Results Eleven studies of 275 adults (mean age, 27 years; 229 men) performed between 1992 and 2016 were included. Most participants had been injured in motorcycle collisions (84%). All studies were at risk of bias, and there were high applicability concerns for the index test (ie, MRI) in four studies given the lack of diagnostic criteria, inadequate descriptions of pulse sequences, and multiplicity of reporting radiologists. Overall, 72% of patients with brachial plexus injuries had at least one root avulsion (interquartile range [IQR]: 53%-86%); meta-analysis of patient-level data was not performed because of sparse and heterogeneous data. With the nerve root as the unit of analysis, 583 of 918 roots were avulsed (median, 55%; IQR: 38%-71%); the mean sensitivity of MRI for root avulsion was 93% (95% confidence interval [CI]: 77%, 98%) with a mean specificity of 72% (95% CI: 42%, 90%). Conclusion On the basis of limited data, MRI offers modest diagnostic accuracy for traumatic brachial plexus root avulsion(s), and early surgical exploration should remain as the preferred method of diagnosis. Published under a CC BY 4.0 license. Online supplemental material is available for this article.