Traumatic brain injuries (TBIs) are a significant health problem, impacting millions of people every year. Although emerging evidence suggests that the composition of the gut microbiome is altered after TBI, no systematic review has been published on this topic. The objective of the present systematic review is to analyze publications that evaluate the impact of TBI on gut microbiome composition. Research articles were pulled from seven databases. The systematic review was performed following the Preferred Reporting Items for Systematic Reviews and Meta-Analyses (PRISMA) guidelines. In order for publications to be eligible for this review, they had to (1) report on original human- or animal-subjects research, (2) evaluate the impact of TBI on the microbiome, and (3) be written in English and (4) be published in a peer-reviewed journal. Of the seven articles that met these criteria, one involved human participants, while the other six reported on experimental animal studies. All studies found changes in the gut microbiome following TBI, with similar changes in bacterial populations observed across studies. The limitations of these studies included the use of primarily male animals, limitations of 16 S rRNA gene sequencing, and small sample sizes. This review was also limited by the small pool of studies conducted in this area. In summary, changes in bacterial populations of the gut microbiome, specifically increases in proteobacteria and firmicutes, were observed across the studies. By evaluating the changes in the microbiome resulting from TBI, potential therapeutic interventions could be explored.